In addition to the normal thermal expansion, another significant contribution on the thickness change of poly(N-isopropylacrylamide) (PNIPAM) ultrathin film has been found under the ambient conditions. With monitoring by multichannel X-ray reflectometry, it is found that the thickness change of PNIPAM ultrathin film can be affected by the moisture in its hydrophilic and hydrophobic state. Furthermore, an asymmetric thickness change has been found in the temperature cycle. Under the ambient conditions, the thickness of PNIPAM ultrathin film can keep stable during the cooling when the temperature is above the lower critical solution temperature (LCST). But as cooling exceed the LCST, the thickness can increase immediately with swelling. If the heating run was conducted after the cooling, the thickness can decrease monotonically, even though the temperature scan is across the LCST.