Abstract: The characteristics of the cross-equatorial air flows and the mass exchange between the Northern and Southern Hemispheric atmospheres are analyzed,and the way of the cross-equatorial air flows is discussed by the statistical analysis of NCEP/NCAR reanalysis data from 1948 to 2000.The mass transport between the two Hemispheres is consistent in 53 years on the whole.The results show that there is a remarkable seasonal variation with the period of six months,and the exchange mostly occur in winter and summer,especially in summer.There are distinct periods of decadal variations but interannual variations act less obviously,except for a 2～5-year oscillation in the 1980～1990s.The anomalous net mass transport to the Southern Hemisphere doesn't vary with the change of season but is dependent on the low-level boreal northeastern trade wind south of the Pacific and Atlantic subtropical highs and the upper-tropospheric northeastern cross-equatorial flow south of the subtropical highs over the Asian monsoon area.However,the anomalous net mass transport to the Northern Hemisphere mostly happens in the summer and is controlled by the southwestern cross-equatorial monsoon,especially by the Somali cross-equatorial jet's northern transport running through the whole layer from low to high levels of troposphere.At the same time,the southeastern lower-tropospheric trade wind over tropical central and eastern Pacific-Atlantic region also plays an important part.