Trimethylphosphate and calcium acetate were added in the polycondensation reaction to form alkali soluble particles in PET. The synthesized PET was converted to a film and treated with alkaline solution to form micro-craters on the surface. Particle formation mechanism was analyzed by FT-IR and the results suggested that phosphonyl group acted as an electron releasing substitute which made a stable coordination complex compound. The particle size of micro-craters was investigated with a scanning electron microscope (SEM). The analysis of the SEM data showed that the size of micro-crater increased as the mole ratio of trimethylphosphate and calcium acetate increased. Initial weight loss temperature and final content of PET were measured by thermogravimetric analysis (TGA). Analysis of thermal stability showed that the phosphorus compound enhanced thermal stability.