Abstract: In the summer 2003, Guangdong experienced an extremely hot weather with long duration of the highest temperature over 35℃. The characteristics of spatial and temporal distribution of highest temperature and their main related causes are studied. The wavelet analysis is used to discuss the low frequency oscillations existing in daily highest temperature in the summer 2003, while the EOF method to study the variation characteristics of the number of days with daily highest temperature over 35℃ in July~August from 1961 to 2003 and their climatic background. The results show that extremely hot weather with daily highest temperature over 35℃ in 2003 happened mainly from early July to the first decade of August, and the number of day was 100 %~200 % more than normal years in most regions except for the Leizhou Peninsula. The high temperature weather was in the positive phase of 60~80 days seasonal oscillation. The subtropical high over the Northwest Pacific is the main synoptic system that produces high temperature weather; when there is also a tropical cyclone near the Philippines, extremely hot weather often happens in Guangdong. The abnormally blocking high on 500hPa in July 2003 over west Ural and Okhotsk, the SSTA in the northern Pacific in December 2002~February 2003 was of El Niño distributing, favouring the intensification of subtropical high over Northwest Pacific and its movement southward and finally controlling the region from south of Yangtze River to Southern China. High temperature weather in Guangdong often happens in May~ October, and mainly in July ~August, very much in the same phase. The extremely high temperature weather occurrences in Guangdong from 1961 to 2003 happened mainly in the early 1960's, the late 1980s~ early 1990's,and the late 1990's up to the present.