Effects of the stereoregularity on the hysteresis in demixing and remixing of the aqueous solution of poly(Nisopropylacrylamide) (PNiPAm) observed as the dissociation of transmittance curves in heating and cooling have been studied. For the liquid-liquid phase separation of the monomer model (N-isopropyl propionamide)/water binary mixture, no hysteresis occurs in demixing and remixing when the scanning rate (v(T)) is small enough. On the other hand, the aqueous solution of an atactic PNiPAm with 46% of the meso content (m) shows the hysteresis even under a very dilute condition (0.01 wt%) at a slow v(T) of 0.01 degrees C min(-1). The difference between demixing and remixing temperatures at v(T) -> 0 systematically increases as m increases. The effect of stereo- regularity is more significant for the cooling (remixing) process of the aqueous PNiPAm solution compared with that for the heating (demixing) process. These results strongly suggest that hysteresis arises from the structure of phase diagrams of the aqueous PNiPAm solution in which the liquid-liquid phase separation and sol-gel transition coexist [F. Tanaka et al. React. Funct. Polym. 2013, 73 (7)]. Above the demixing temperature (T-dmx), a dilute and a concentrated PNiPAm solutions appear, and the concentrated one of which solution undergoes physical gelation. The physical gel formed at the polymer-rich region returns to a sol by cooling at the remixing temperature T-rmx which lies possibly at lower than T-dmx. Thus, the hysteresis in demixing and remixing is observed.