The thermal stability of nucleation effect of three highly effective beta-nucleating agents, namely an aryl diamide-based nucleating agent (TMB-5), a rare earth complex (WBG-II), and calcium pimelate (Ca-Pi) in isotactic polypropylene (iPP) was investigated by DSC multi-scanning cycles. All three nucleating agents induced iPP to form high contents of beta-crystals. After several thermal cycles, the relative content of beta-phase crystals (K.sub.DSC) in the nucleated iPP was decreased and the crystallization peak temperature ( Formula omitted) was also decreased. The K.sub.DSC and Formula omitted of iPP nucleated with TMB-5, WBG-II, and Ca-Pi were decreased by 0.0477, 0.1257, 0.0603, and 0.97, 2.71, 1.18 °C, respectively. It indicated that the nucleation effect became worse. The nucleation effect of TMB-5 decreased less while that of WBG-II and Ca-Pi was significantly reduced. Thus, the orders of thermal stability for nucleation effect of these three nucleating agents were TMB-5 > Ca-Pi > WBG-II.