Using the hourly meteorological data from Menyuan and Qilian weather stations from 2004 to 2013, the diurnal change characteristics of orographic clouds are analyzed. It is found that the daily changes of total and low cloud amount show a pattern of two peaks in summer clearly, and the daily change of stratocumulus is opposite to that of cumulonimbus, and the occurrence frequency of stratocumulus is the greatest in the morning. The occurrence frequency of stratocumulus is greater in Menyuan than in Qilian, but that of cumulonimbus is greater in Qilian than in Menyuan. The circulation of mountainvalley breeze in both stations shows same variation characteristics: maximum wind velocity in the afternoon and minimum in the early morning, but the wind speed is greater in Qilian than in Menyuan. The dominate time of valley breeze is longer than that of mountain breeze in Menyuan, and vice versa in Qilian. The occurrence time of cumulonimbus is closely related to the occurrence time of maximum mountain breeze or valley breeze, which indicates that the mountainvalley breeze is a key factor in the course of orographic cloud formation. Under the control of a weather system, cumulonimbus maintains much longer with more precipitation; however, under the control of orographic winds and thermal turbulence, it maintains shorter with less precipitation. The formation of stratocumulus is closely related to the circulation of mountainvalley breeze and diurnal change of the boundary layer, and can be distinguished as three types: transformed from altostratus, derived from cumulonimbus when its growth is limited, resulted from the local circulation of mountainvalley breeze.