This research is to use principle of the polarized light to self-made polarizing microscope, use starch grains of potato in different directions of the polarized light have double refraction to come to examine the starch grains of potato. A kind of method to examine that starch grains or double refraction, because the structure is simple and easy and convenient. Observation use the polarizer to change state of light with microscope, and observation starch grains in the rotation angle difference between the polarizers, and observation the difference between the use of polarizer and use of iodine solution and discussion the principle.