Changes of the climatic zones are related to climate warming and its impacts. The northern boundary of sub-tropical zone is between the temperate and sub-tropical zones, which is a very important boundary in China's climatic divisions. In this paper a case study is made on the impact of climate warming on the northern boundary of sub-tropical zones of China. The data covering about 55 years (1951~2005) of daily mean temperature with 740 stations are obtained from China Meteorological Administration. According to the definition of the first class climatic zone from Climatological Atlas of the People's Republic of China, the authors use the number of days with secular pentad mean temperature passing through ≥10℃ as the main index, and January mean air temperature as the auxiliary indices. Namely the definition of the northern boundary of sub-tropical zone is the divide where the number of days with secular pentad mean temperature passing through ≥10℃ reaches 218, the accumulated temperature with ≥10℃ is 4500~4800℃, and January mean air temperature is 0℃. The authors count respectively the inter-decadal changes of indices of the first class climatic zone in north sub-tropical and temperate zones in 1951~2005, and the changes of the north sub-tropical belt in the periods 1951~1970, 1971~1990 and 1991~2005. The results show that: the number of days of secular pentad mean temperature passing through ≥10℃, the accumulated temperature ≥10℃ and January mean air temperature all have an upward trend in the 55 years in the vicinity of 34 degrees north latitude. Since the 1950s, there has been little change in the western section of the northern boundary of sub-tropical zone; the central and eastern sections of the northern boundary of sub-tropical zone moved to the north over a large range. By the beginning of the 21st century the central and eastern sections has moved to near 35°N, i.e., 2~3 degrees of latitude northward, compared with the period 1951~1970. The northern boundary of sub-tropical zone moving towards the north, which is a response to climate warming, will have a very important influence on the agricultural pattern and the ecological environment.