Abstract: Interannual variations of moving tracks of tropical cyclones (TCs) over the Northwest Pacific during the period of 1959-2003 and their physical mechanism are analyzed by using the typhoon data provided by JTWC, the wind field data from the NCEP/NCAR reanalysis dataset, and the sea temperature data provided by the Scripps Oceanographic Institute, U.S.A. . Analysis results show that interannual variations of the cyclogenesis and track of TCs over the Northwest Pacific are very obvious and closely associated with the thermal states of the Western Pacific, especially with the subsurface sea temperature of Western Pacific warm pool. When the Western Pacific warm pool is in a warm state, then the moving tracks of TCs will shift westward over the Northwest Pacific, thus, the number of typhoons influencing China may be above normal. On the contrary, when the Western Pacific warm pool is in a cold state, then the moving tracks of TCs will shift eastward, thus, the number of typhoons influencing Japan may be above normal, but the number of typhoons influencing China may be below normal. This analysis result is further demonstrated with the difference between the moving tracks of TCs over the Northwest Pacific in 2004 when the warm pool was in the cold state and in 2006 when the warm pool was in the warm state. Moreover, the physical mechanism of the impact of the thermal states of the Western Pacific warm pool on the moving tracks of TCs over the Northwest Pacific is revealed by the dynamical analysis of the influence of the thermal state of the warm pool on the monsoon trough position and the transformation from the mixed Rossby-gravity wave to the ropical depression-type disturbance, i. e. , the TD type disturbance over the tropical Pacific. When the Western Pacific is in a warm state, then the monsoon trough shifts westward, thus, the convergent region of zonal wind field in the southern part of the monsoon trough also shifts westward over the Northwest Pacific. Therefore, in this case, the region of the transformation from the mixed Rossby-gravity wave to the TD type disturbance also shifts westward, which leads to the westward shift of TCs geneses and their moving tracks. On the contrary, when the Western Pacific is in a cold state, then the monsoon trough shifts eastward, thus, the convergent region of zonal wind field in the southern part of the monsoon trough also shifts eastward over the Northwest Pacific. Therefore, in this case, the region of the transformation from the mixed Rossby-gravity wave to the TD type disturbance also shifts eastward, which leads to the eastward shift of TCs geneses and their moving tracks over the Northwest Pacific.