Based on the Bayesian mutation analysis, it is found that the frequency of late-season (October-November) typhoon declines strikingly during 1998-2016 compared with 1979-1997. This trend is found in most parts of Western North Pacific (WNP), and is most significant in the southeastern sub-region of the WNP (0°-17.5°N,135°-180°E). The Genesis Potential Index (GPI) also exhibits salient decrease in this region. The contributions of the four environmental parameters (absolute vorticity, vertical wind shear, relative humidity and maximum potential intensity) to GPI were assessed separately. Results indicate that the decreaing of late-season typhoon frequency during 1998-2016 is mainly driven by the change of dynamical factors (absolute vorticity and vertical wind shear).