The quantitative impact in terms of moisture supply of typhoon Goni on heavy rainfall of typhoon Morakot in Taiwan has been studied in the paper using the weather research and forecasting (WRF) numerical model based on NCEP/NCAR reanalysis data.The numerical analysis indicates that the intensity of typhoon Morakot was enhanced by continuously positive vorticity and water vapor supply from typhoon Goni that kept the persistent power of typhoon Morakot during its lifetime in Taiwan.The results of the simulation are consistent with the observations.Furthermore,a specific experiment of WRF numerical simulation in which typhoon Goni was filtered out was carried out,which shows that the typhoon Goni contributed about 30% in power and water vapor to the heavy rainfall of typhoon Morakot in Taiwan.