Abstract: Part-gelatinization Sizing Technology (PGST) is a novel technology to size yarns by using part-gelatinized starch sizing agents in a medium temperature conditions. In this paper, the microstructure of sizing membrane formed both on the yarn surface and in the cross-section of the sized yarns are examined by using SEM, the mechanism of the formation of sizing membrane on the surface of yarns in PGST are thus proposed. It is found that sizing slurry prepared in PGST contains a certain amount of fully-, or partially-gelatinized starch sizing agents and a greater amount of not gelatinized starch particles; the gelatinized starch sizing pulp has low viscosity is easier to penetrate into voids inside yarns while the not gelatinized starch particles are superabsorbent against water deposited on the surface of the yarn. During drying process, the saturated, not gelatinized starch particles deposited on the surface of the yarn to form sizing membrane jointed with the gelatinized starch inside the yarns. Such a complex membrane structure covered the whole fiber surface within and on the surface of the yarns. Key words part-gelatinization sizing technology (PGST); mechanism of sizing membrane formation; starch sizing materials