Abstract: In order to invest igate the effect by the meteorolog ical factors from lower latit udes on Meiyu/Baiu front,a diagnostic analysis for the two cases during June to July of 1985 and 1986 was examined.We found that.(1) when the tropical convective activity westward moving near 10-17°N,140°E from east side(10-17°N,160°E),the northw ard shift of the Meiyu/Baiurain beltoccur.(2)the onsets of Meiyu in China of both cases took place just when the convective activity around lower lat it udes moved westward through～140°Ewith the 20-30 days oscil ation period at the first time in June.(3)the main fact or which results in the northward and southward shift of Meiyu/Baiurain belt is thought as the 20-30 day oscillation of the tropical convective activity around West Pacific.(4)the Rossby wave propagation which is excited by the convective activity around Philippine occurred only in late summer and did notgive anyim pact to the intraseasonal variation of the Meiyu/Baiu rain belt in the both cases.