Yunnan Province, China, has its unique "cubic climates" which means that the climates in the region vary greatly. There are seven meteorological stations in the province which have measured data of global solar radiation on horizontal surfaces, and only four of them (Kunming, Tengchong, Hekou and Jinhong) have measured data of diffuse radiation on horizontal surfaces. With these data and the relevant data of the geographic and meteorological parameters of these four stations, we develop, in this study, two kinds of correlations to estimate the monthly average daily diffuse radiation on horizontal surfaces in Yunnan Province. It is believed that these correlations are applicable at any site in Yunnan Province.