In this study, we collected Taiwanese longline (LL) fishery data and environment variables during the period of 1998- 2004 to investigate the relationship between LL catch data of yellowfin tuna (YFT) and oceanic environmental factors using a principal component analysis (PCA). Results of the PCA showed that monthly variations in catch per unit effort (CPUE) values were significantly correlated with the sea surface temperature (SST), subsurface temperature at 105 m, thermocline depth (horizontal) gradient magnitude, chlorophyll-a concentration, and fish size. April and May were the warmest months of the year in terms of the SST, and the thermocline was generally deep. After July, a drop in the temperature below the preferred temperature range for YFT is probably the reason that the CPUE subsequently decreased in the period of 1998-2003. It was suggested that the CPUE by age at a given time was significantly affected by chlorophyll-a concentrations 1-3 months prior to that time. The lower thermocline depth gradient magnitude enhanced the aggregation density of YFT in 2004 which showed that the high catch and high CPUE of the YFT fishery increased from the western to the eastern Arabian Sea.