Objective To research for the effective time of ultraviolet germicidal irradiation for air disinfection in a low temperature. Methods A clinic abortion rooms (type Ⅱ environment) and a clinic surgery diagnosis rooms(type Ⅲ environment) were selected randomly for our experiment and sampled in them after 30 min, 60 min, 90 min and 120 min disinfection, both in the temperature below 20 ℃ and of the relative humidity below 60％. Every sampling involved 30 air samples and all the experiment procedures were in accordance with the technical standard for disinfection established by Ministry of Health in 2002. Results As our data showed, in a low temperature (ambient temperature ＜ 20 ℃), there was difference in colony count after 30 min and 60 min disinfection in the type Ⅲ environment (t = 2. 603, P ＜ 0. 05), and there was difference in colony count after 90 min and 120 min disinfection in the type Ⅱ (t = 0. 373, P ＜ 0. 05), there was no statistically different in other time between the two groups, did the colony count of air bacterial have clear statistical difference (X2 = 0. 517, P ＞ 0. 05), the eligible count in each time was similar (P ＞ 0. 05).Conclusions The effective time of ultraviolet germicidal irradiation for air disinfection in a low temperature (ambient temperature ＜ 20 ℃) is about 30 to 60 minutes.