Abstract: Abstract: With the rise of total engine power and population, agricultural machinery has been considered an important pollutant source of the deterioration in air quality in China. However, few studies have focused on agricultural machinery emissions. In this case, it is difficult for the policy makers and researchers to understand the actual emission levels of agricultural machinery in China. To analyze real-world emission characteristics of agricultural machinery, twelve large and middle-sized agricultural tractors were tested by using a portable emission measurement system (PEMS) in this study. The typical operating modes were categorized as idling mode, moving mode, and tillage mode. The results showed that most of the emission peaks occurred on tillage mode, which can be attributed to the variable engine loads of tillage mode. Hourly emission factors of CO，HC，NOx and PM were higher for tillage operating mode than those for idling and moving modes - especially for PM. Hourly emission factors of PM for tillage mode were 52.6 and 63.5 times higher than it was for idling and moving modes. Due to the higher fuel consumption, fuel-based emission factors of CO, HC and NOx for tillage mode were lower than those for idling and moving modes. In this study, we define the tractors before 2007 model year as the Pre- China I machinery. Compared with those of China I Stage and Pre-China I Stage emission standard tractors, gaseous emissions and PM emitted from China II emission standard tractors were lower. The results indicated that from China I Stage to China II Stage, the reductions of average hourly emission factors of the four pollutants were 31.49%，5.96%，6.17% and 5.91%, respectively. Those results also demonstrated the effectiveness of emission control regulation and the improvement of emission control technology. The NONROAD model, which was developed by the Environmental Protection Agency of the United States of America, is widely used to calculate the emission inventory of NONROAD mobile sources and evaluate the emission levels of NONROAD machinery. In this study, we compared the measurement results with the emission factors from the NONROAD model. Results showed that the emission control levels of HC, NOx and PM emissions in this study lagged behind those of the U.S. For instance, HC and PM emission factors from this study are equated with those from model years of 2000 and 2006 on the NONROAD model, while the NOx emission factor from this study is higher than it was from the model year of 1998 on NONROAD model. Extensive work was undertaken in Shanxi Province to obtain the annual working hours of large and middle size tractors. Results show that annual working hours of tractors are 400 - 430, which is similar with the data on the NONROAD model. Based on the population and annual working hours, we estimate the total emissions of CO, HC, NOx and PM from large and middle size agricultural tractors in 2011 were 32.3, 10.6, 81.0 and 9.1×104 tons, respectively. The data indicates that large and middle size agricultural tractors are an important source of air pollutant in China compared with total emissions from motor vehicles.