Poly (N-isopropylacrylamide) (PNIPAM) and its hydrogels were synthesized trom the aqueous solutions of N-isopropylacrylamide (NIPA) by radiation-induced polymerization, combined with crosslinking of the growing polymer chains with 60Co gamma source at Hanoi Irradiation Center. Degrees of polymerization and gelation depend on initial concentration of irradiated solution and radiation dose. Conversion yield to polymer increased with the NIPA amount and reached a maximum when NIPA exceeds 10 percent. Crosslinking density, i.e, gelation ratio also increased with monomer concentration and radiation dose. 10 percent NIPA solution was irradiated at different doses in low dose range and the gel dose (Dg, dose for incipient gel formation) was extrapolated to be 178 Gy. The influence of temperature of swelling media on the swelling properties of the hydrogel was investigated trom room temperature to 45°C. The results suggested that the obtained hydrogels are thermoresponsive with an apparent transition trom hydrophilic to hydrophobic state in around 31.5 to 33°C. Phase transition temperatures of hydrogel, which usually knows as its lower critical solution temperature (LCST) slightly reduced with increasing of radiation dose.