Thermodynamic transition temperature of crystal forms is essential information for the formulation developmental studies. However, determination of the transition temperature is frequently time and labor consuming. Herein, solvent-mediated transformation, which is a very powerful method to determine the transition temperature, was combined with thermal analysis to offer a convenient and rapid method to estimate the polymorphic transition temperature. The thermodynamic transition temperature of sulfamerazine, which cannot be obtained by conventional DSC measurements, was investigated using this procedure. The transition temperature obtained by this in situ transformation technique was almost consistent with the thermodynamic transition temperature.