This article of examines the starch-grains of the potato using the polarized light principle by electron microscopy. We mince the potato to obtain the starch-grains of potato under the different temperature, and to define the gelatinization temperature. The experiment show that the gelatinization temperature is starting in about 60℃, and the obvious gelatinization when the gelatinization temperature in 70℃. This is a kind of method to examine that starch grains or double refraction.