H-1 MAS NMR spectroscopy has been applied to study the urea effect on phase transition of two similar thermosensitive polymer hydrogels: poly(N-isopropylacrylamide) (PNIPAM) and poly(N,N-diethylacrylamide) (PDEA). It is found that urea influences the phase transition of the hydrogels in opposite ways: lowering the lower critical solution temperature (LCST) of PNIPAM and hence stabilizing its globular structure, whereas raising the LCST of PDEA and destabilizing the globular structure. The self-diffusion coefficient and urea-polymer nuclear Overhauser effect (NOE) measurement reveal that urea has a stronger interaction with PNIPAM than with PDEA. Moreover, the enhanced positive water-PNIPAM NOE suggests that urea not only interacts directly with PNIPAM via hydrogen bond but also intensifies the hydrogen bonding interaction between water and PNIPAM. We suggest that different urea-polymer hydrogen bonding interaction due to the presence or absence of amide hydrogen is correlated with the distinct LCST variation of PNIPAM and PDEA.