A novel thermosensitive polymer, poly(methyl 2-propionamidoacrylate) (PMPA), was prepared and the phase transition behavior in an aqueous solution was studied. PMPA have geminal substituents of propionamide and methoxy carbonyl in each monomer unit. It was found that PMPA shows the lower critical solution temperature (LCST) at 50.6 degreesC sharply. The LOST of PMPA was almost independent of the polymer concentration above 40 g/l, while, below 40 g/l, it decreased with the increasing polymer concentration. The effect of salt addition (NaCl, NaBr, and Na2SO4) On the LCST was also studied. The LOST of PMPA linearly decreased with the increasing concentration of each salt. The salting-out effect of these salts was in line with the Hofmeister series: Na2SO4 > NaCl > NaBr.