Abstract: In this paper, two cases of low level, southeastly jets in North China are compared. It is found that whether there will occur heavy rain near the jet or not depends on its structure. There are marked differences between them in moisture transfer, three-dimensional flow and stability of the jets. In the forecasting of rainstorm, it is insufficient only to pay attention to the strength of wind, even with a remarkable horizontal and vertical shear. Favorable thermal and dynamic condidons are necessary for rainstorm.