To explore the stability of impact assay for determination of microorganism in indoor air, the comparisons of continued sampling times of 2-minute and 4-minute for monitoring on microorganism of indoor air were carried out with JWL- Ⅱ B multifunctiom air microorganison monitor in 11 hotels, public places of entertainment, barbar shops and beauty shops. The results showed that, under a same environmental condition, there was no significant differences between the data obtained by 2-minute monitoring and 4-minute monitoring (t=1.854, P＞0.05).It indicated that impact assay presented very good stability in detecting the microorganism of indoor air within certaindetecting duration.