Abstract: In recent years, smog has emerged in most of major cities in China. Epidemiological and toxicological studies showed that the ultrafine particles in the atmosphere adhere to a large number of toxic substances, which can cause serious harm to human respiratory and cardiovascular and other organs after they enter the human body through the respiratory tract. and the data indicated that it had a significant correlation with human morbidity and mortality. Particulate emissions from diesel engines are an important source of ultrafine particles in urban atmosphere. The researchers found that the weighing method of particulate matter detection cannot accurately reflect the emission levels of internal combustion engines. This new method for measuring the number of particulate matter emissions after removing volatile matter from the method has been widely recognized and included in the scope of regulatory investigation. However, there are few studies on the number and size distribution of particles in the transient process of diesel engine. In order to simultaneously control the quality and quantity of particulate matter emitted from heavy duty diesel engines, the particle distribution of exhaust particulates from diesel engines under steady and transient conditions was studied. The test bench is a 12 L heavy duty diesel engine and an electric dynamometer. The test bench is a prototype with no post-processor and meets the Euro IV regulations. It has been certified by China Automotive Technology Research Center (CATRC). The engine intake system consists of a two-stage turbocharging system with variable geometry section turbine (VGT), a high-pressure EGR system and an intake valve late closing system (IVCT system, which is developed by the research group independently). Cambustion DMS500 fast particle spectrometer was used to analyze the particle size distribution of diesel engine under transient conditions. The experimental results showed that, in the steady state, with the increase of load or speed, the concentration of accumulated particles showed an upward trend, corresponding to the increase of count median diameter (CMD), the surface area and mass of accumulated particles showed an increasing trend, and the peak value shifted to the direction of large particle size. In this study, the total number concentration of particles during the steady state is 1.5×106-4.5×106/cm3, and accumulation mode particle was 2×106-1×107/cm3, nucleation mode particle was 1×107-3×107/cm3 which account for 65%-96%. In the steady state, the number of nucleation mode particles is the main source of the total number of particles. The equivalent ratio of all steady state conditions is less than 0.7. The effect of equivalent ratio on particle distribution is obvious at middle and low rotational speed, but weakens at high rotational speed, especially at high load. The European Transient Cycle (ETC) was used for the transient test. In transient condition, the spikes also appear in the number concentration of particles which similar to that of quality of particulate matter, and the number concentration peak even reaches 2×108-7×108 /cm3, which is 2 orders of magnitude higher than that of the corresponding steady state operation. The number concentration of nucleation mode particle increases significantly in the spikes, but the proportion of the number concentration of nucleation mode particle in the total particles is reduced. The number concentration of accumulation mode particle is the main part of the spikes which is different from the steady state condition. The particle size peak is mainly concentrated in the 50-200 nm range at accumulation mode, the 6-8 nm and 20-50 nm at nucleation mode, this mainly because that the equivalent ratio in the transient process appears to exceed the critical equivalent ratio of 0.8 working conditions. The equivalent ratio does not exceed 0.7 in steady state operation, but the condition corresponding to the emission peak appears to exceed the critical equivalent ratio 0.8. The results are of great significance for particulate emission control and generation mechanism of heavy-duty diesel engine, and can provide data support for matching calculation of post-exhaust processor.