The microstructures of ethylene random copolymers are evaluated using DSC fractionation by the degree of crystallinity. This method is based on the same principle of separation as temperature-rising elution fractionation (TREF) which uses a crystallization/dissolution procedure; however it does not physically separate the fractions. The resulting copolymers are sorted by crystallite sizes : the crystallite size and lamellae thickness are directly related to the ethylene block length. Copolymers including E/VA, E/nBA, E/nBA/CO, E/nBA/VA, E/MAA, E/AA and LLDPE have been analyzed by DSC fractionation to elucidate structural differences. We see that (a) specimens with high comonomer content are less crystalline; the ethylene segments are shorter, (b) the comonomer type makes a difference in fractionation if H-bonding is involved, (c) the narrow comonomer distribution, i.e. narrow ethylene-segment-length distribution, has fewer DSC fractions; the broad distribution has more fractions, (d) fractionation by crystallinity is affected if the molecular weight/viscosity is very high, (e) branch content reduces the crystallinity and shortens the ethylene blocks, and (f) there is no direct relation to molecular weight distribution or comonomer content.