There was a heavy high temperature and drought event and disasters in a large area of Yunnan Province in the summer of 2003, it is one of the heaviest ones in Yunnan since 1961, its influencing area, sustaining duration and harm extent, is the second place except 1983. It is a summer extreme climate event since the mid 1980s during which Yunnan entered a warming phase. The summer drought disaster has been diagnosed by using data of height fields at 500hPa, Outgoing Longwave Radiation (OLR) and Temperature of Black Body(TBB) at cloud top. The main causes that lead to the persisting high temperature and drought event in Yunnan are found that the position and intensity of the Western Pacific subtropical high at 500hPa consistently located to the west and stronger than average and the monsoon depression also located to the west and weaker than average respectively. TBB and OLR values over Yunnan are greater than the average. Rainfall cloud system is seldom, and the convective activities is inhibition.