The crystallinity of micronized starch and the effect of particle size of starch on the properties of thermoplastic starch were studied. It was found that the starch was destroyed by mechanical treatment. However, starch particles join more easily as the size of starch decreases. It was found that when the particle size of starch was suitable, the properties of thermoplastic starch were improved. The thermoplastic starch was prepared by micronized starch of d₅₀=4.23 μm. Its mechanical property, rheological property and degradation were excellent. Its tensile strength was 21.7 MPa, and the breaking elongation was 138.3%. It was also concluded that its biodegradation ratio was close to 60% in the soil after 28 days.