The previous genesis potential index (GPI) can be applied to accurately simulate the climate distribution of tropical cyclone (TC) genesis,yet performs poorly in the simulation of interannual variation of TC genesis.The present study attempts to improve the GPI over the western North Pacific (WNP),by considering the important impact of relative vorticity on interannual variation of TC genesis.Based on the best-track data from the Joint Typhoon Warning Center and NCEP/NCAR reanalysis data from 1979 to 2011,we substitute the absolute vorticity in the previous GPI with a modified relative vorticity.The Coriolis factor remains intact.The difference of tropical cyclone formation between the South China Sea (SCS) and WNP is also considered,and the GPI for the SCS (5°-25°N,100°-120°E) and WNP (5°-40°N,120°-180°E) are respectively developed.The modified GPI is shown to improve the simulation of the climate distribution of TC genesis.Moreover,compared with the previous GPI,the modified GPI greatly improves the simulation of the interannual variation of TC genesis,in particular in regard to weak tropical cyclones.