Using the monthly latent heat flux data and relevant meteorological variables for the period of 1958 to 2014 produced by the Objectively Analyzed Air-Sea Fluxes project (OAFlux) of the Woods Hole Oceanographic Institution, and surface pressure data extracted from NCEP/NCAR reanalysis data, the authors investigate the long-term trend of latent heat flux over the West Pacific-South China Sea (WP-SCS). The method of Trend-EOF is applied in this study. An increasing trend of latent heat flux is found when the entire area is taken into account as a whole, especially in the winter. A north-south contrast of latent heat flux changing trend in the winter is noted, especially in the SCS where an increasing trend occurred in the north and a decreasing trend occurred in the south. The increasing trend of latent heat flux in the Philippine Sea and northern SCS is primarily attributed to an increase in the air-sea humidity difference. The decreasing trend of latent heat flux in the southwestern SCS is resulted from a decrease in the air-sea humidity difference, while in the southeastern SCS it is a result of the decrease in wind speed. Air-sea humidity difference works together with wind speed, leading to the north-south contrast of long-term changing trend. In addition, the trend of wind speed is closely associated with changes in surface pressure, while the trend of air-sea humidity difference is associated with changes in sea surface temperature.