Abstract: In this paper we have analysed the 500 mb flow patterns from May to July, 1954-1962, to decide the Mai-yü period of the lower and middle Yantze.It is found that the variations of the subtropical high of Western Pacific and the westerlies over East Asia are the essential elements to determine the beginning and the end of Mai-yü. Especially, the variation of the ridge line of the subtropical high in the region 110°-125°E is more important in determining the Mai-yu period. If the ridge line moves across 20°N from lower latitude, Mai-yü will begin; if it moves further north- ward and crosses 25°N, Mai-yü will terminate. The variation of westerlies on 500 mb is another index. The disappearance of the subtropical jet and the sudden northward push of the easterly along 90°E indicate exchange of seasons. After this variation Maiyü will begin when the subtropical jet in the band of longitudes 115°-125°E retreats northward to the north of 30°N and terminates when the jet acrosses 35°N.