Abstract: Heating load of a greenhouse heating system is directly related to local outside temperature. Determination of the temperature in greenhouse design is very important for guaranteeing plant growth, decreasing heating equipment investment, and thus for saving energy consumption and improving grower’s economic benefit. On basis of analyzing national and industry standards and using international experiences for reference, this paper concluded that the outside temperature in greenhouse heating system design should use the average minimum temperature over a given periods. They had 99%±0.5% of assurance ratio that the outside temperature was higher than the calculation temperature for greenhouse heat load design. As a guide of the methodology, contour maps of average minimum temperature in the latest 30 years and 20 years were plotted according to the national climate records.