Phase transition occurring in three different types of poly (N-isopropylacrylamide) gels-normal cross-linked gel, comb-type grafted get, and comb-type grafted gel with styrene-modified comb chains-has been investigated by variable-temperature measurements of H-1 NMR spectra and spin-spin relaxation time. Three different gels exhibit distinct collapse behaviors in response to increasing temperature. For the normal gel, remarkable network shrinking occurs in a relatively narrow temperature range from 32 to 35 degrees C. For the styrene-modified comb-type gel, overall chain shrinkage appears in a very broad temperature range from 22 to 35 degrees C in which the styrene-modified comb chains shrink at lower temperatures (22-32 degrees C) than the backbone networks (32-35 degrees C). In the comb-type gel without styrene modification, however, the backbone networks shrink first (at 32-35 degrees C) on heating, followed by collapsing of comb chains (at 35-36 degrees C). During shrinkage of backbone networks the comb chains are expulsed from the main gel networks which is revealed by abnormal T-2 increase of comb chains. T-2 measurements also reveal that the styrene-modified comb-type gel in the equilibrium swelling state has more rigid network structure than both conventional gel and comb-type gel without styrene modification.