Two commercial ethylene—propylene random eopolymers(A and B)for hot water piping of which A has better long term heat stability and slow crack growth resistance performance than B were fraetionated with respect to the isotactitv distribution by preparative temperature-rising elution fractionation(TREF)．The molecular wetght and distribution of fractions and the original resins were investigated by gel permeation chromatography．The results indicated that both A and B had broad isotacticity distribution and A had fewer portions of high isotacticity than B．At the same time A had broader and more typical double—peak molecular weight distribution．The fractions of A had higher malecular weight and broader molecular weight distribution than those of B’s．All of these results implied the differences between the catalysts or the polymerization processes for the two copolymers．