Abstract: By using 1982 ～ 1994 NCEP/NCAR wind data, the at mospheric flow fieldis decomposited into two parts:the barotropic one, i.e.the vertical mass-weighted integral, and the baroclinic one, i.e.the difference between the observed winds at a certain level and the barotropic component; The dist ribution feature of barotropic/baroclinic flow field in winter/summer are investigated.Monsoon is classified into three types based on the baro tropic/baroclinic flow -patterns at the lower troposphere.It is pointed out that the seasonal variation in the circulations of baroclinic and barotropic components are responsible for the reversal of surface winds between winter and summer, thereby for generating monsoon.The baroclinic flow field is a thermally-driven circulation which is mainly caused by land-seat hermal contrast, while the barotropic flow field is mostly a dynamically-driven circulation.So such a decom position is helpful for under standing the causes of monsoon.Results also show that the monso ons over tropical Asia, Africa and South America are barotropic flow -pattern, while the monsoons over subtropical Pacific in the Northern and Sout hern hemisphere are of baroclinic flow -pattern and the East Asia subtropical monsoon is of barotropic/baroclinic mixed flow -pattern.