High-density polyethylenes with unimodal and bimodal molecular weight distribution have been fractionated according to crystallizability using preparative temperature rising elution fractionation. The molecular structure and thermal properties of the fractions with their whole polymers have been characterized. The average short chain branching content of the fractions obtained ranged from 0 to 8 branches per 1000 carbon atoms while that of the whole polymers is about 2 branches per 1000 carbon atoms. The bimodal resins have a slightly higher frequency of short chain branch in higher molecular weight species than in those of the unimodal resins. The short chain branching distribution as well as the low molecular weight species in the fractions seem to be important parameters to determine thermal behavior of the fractions. The fractions with the short chain branching content above 3 branches per 1000 carbon atoms showed a significantly different thermal behavior from those with less than 3 branches per 1000 carbon atoms.