Submicron-sized N-isopropylacrylamide (MPA)-ethyl N-acryloylglycine (ENAG) copolymer gel particles with various compositions were prepared by precipitation polymerization in water. Their swelling behaviors were determined by photon correlation spectroscopy(PCS). Our results showed that the volume transition temperature of copolymer gel particles decreased with increasing the mole ratio of ENAG. In model development, we modified the extended Flory-Huggins theory as a free energy of mixing term and employed the modified Flory-Rehner theory for the elastic contribution and compared the proposed model with our swelling data. The model predicted fairly well the swelling behaviors of copolymer gel particles.