The present invention relates to a vinyl chloride polymer having good thermal stability due to the restraint of dehydrochlorination by heat or ultraviolet rays, and a method of preparing the same. The generation of the dehydrochlorination of the vinyl chloride polymer due to heat or ultraviolet rays may be markedly restrained, the thermal stability thereof may be improved, and the discoloration thereof or the modification of the physical properties thereof may be prevented. In addition, a modifier may be introduced to a polymerization process at the end of the polymerization, and high thermal stability may be attained without generating the transformation of the vinyl chloride polymer.