A simple thermodynamic model, based on an extension of Flory-Huggins theory, is applied to temperature rising elution fractionation (TREF). Dependence of the fractionation process on melting temperature, melting enthalpy, average crystallinity, average crystallizable sequence length, and polymer-solvent interaction parameter is predicted. Results from the model fit experimental TREF data, and correctly predict number-average branch points for TREF fractions.