The temporal variation of PM_2.5 concentration in Beijing, Shanghai and Lhasa in 2015 is investigated. Then, the analyses on their seasonal variation and comparison of the relative importance of diurnal variation and daily variation are conducted. Results show that the PM_2.5 concentration is generally higher in winter than that in summer from a long-term view. From a short-term view, the variation of PM_2.5 concentration in Beijing and Shanghai mainly demonstrates interdiurnal feature, and the synoptic systems exert profound impacts. The wintertime PM_2.5 concentration in Beijing also displays diurnal variation, but it is not evident in summer. In Shanghai, the diurnal cycle is not significant in both winter and summer. On the contrary, the synoptic system over Lhasa is stable. The daily variation is weak, while the diurnal variation is strong. The variation of PM_2.5 concentration is hence featured by two peaks.