The allotropic transition of polymorphsum in tristearin is followed by X-ray diffraction. Four modifications; α-, β’-, intermediate and β- form of tristearin had been identified. The transition rate of modification is depended on a quantity of impurity (liquid paraffin) mixed in tristearin. In the much quantity of impurity, the transition rate of modification become the rapid transition of α-form to β-form. The intensity of X-ray diffraction on (001) and (100) plane in a unit cell of β-form is the same phenomenon for impurity effect.