The initiation，propagation and breakdown of crazes in uhrathin film of a-phase isotatic polypropylene with different molecular weight during slow tensile process in TEM have been observed．The local orientation of lamellea in spherulites changed the straightforward propa-gation direction of crazes．Most of crazes irdtiated in the spherulites．The microfibril break-down became easier as the molecular weight decreased，and most of the fibril breakdown took place in the middle．It was different from that in PS where fibril breakdown occurred in the strain softened zone at the craze-bulk interface．