The severe flood and drought occurred in Yunnan in May 2001 and May 2005,respectively.In this paper,the comparison between the two events has been conducted. It can be seen,through analysis and study,that there existed very different characteristics in the circulation,early or later about monsoon onset and ENSO events.At middle troposphere in May 2001,East Asian trough lean to west and more stronger,and southern westerlies trough was stationary as well as stronger, made the cold air and warm air meet in Yunnan easily,and caused heavy rainfall.In May 2005,East Asian trough lean to east and southern westerlies trough weaken,which induce to water vapor and cold air and warm air,not meeting in Yunnan.These were the possible reasons for flood and drought in the Yunnan area.Meanwhile,the monsoon onset and its strength were associated with precipitation in Yunnan in May.The monsoon onset was early and it was stronger in May 2001,on the contrary,in May 2005.So the precipitations between two years were quite different.In addition,the analysis of the water vapor transportation in Yunnan showed,the water vapor was convergence in Yunnan in May 2001,and it was more than that in normal years,the direction of moisture flux was southwest and strong.But the water vapor in Yunnan in May 2005 was divergence,the water vapor was transported from the Bay of Bengal to South China Sea,and it was very dry in Yunnan.It can also be found from the analysis for ENSO,the flood in May 2001 and drought in May 2005 were related with the sea surface temperature in the last several months.