Abstract: Crop straw burning, an important category of biomass burning, contributes significantly to airborne fine particles. This work is to characterize PM_2.5 emission factor of this source. For this purpose, a system of combustion simulation-dilution tunnel sampling was developed and PM_2.5 was measured during the combustion of wheat, corn and rice straws collected from five grain producing areas, including Zhejiang, Sichuan, Henan, Hebei and Beijing. The results showed that PM_2.5 emission factors from flaming simulation were 7.2 to 39.0 g/kg comparable with those from field burning, implicating that these two combustion conditions would be similar. Moreover, 2.4～11.5 times higher PM_2.5 emission factors were observed in smoldering combustion, indicating that combustion status would affect emission features remarkably. There were also novel differences in PM_2.5 emission among different crops. However, the difference of emission factors among the straw from different regions was relatively small.