The microdomain structure and domain packing mode in polydisperse PMMA-b-PTHF diblock copolymer have been'studied with SAXS and TEM techniques The copolymer composition is 48/52

and its heterogeneity index is 1.99. We found that the morphology of domain is spherical micelle which is also heterogeneous in size

instead of lamellar microdomain. The mode of microdomains packing is nearly disordered arrangement instead of macrolattice. Our results show that the theory on the morphology formation of monodisperse block co-polymer is not alwayssuitable for thepolydisperse block copolymer.