Yellowfin tuna is the main target species of longline fishery in offshore waters of Taiwan. The main fishing ground from southwestern waters of Taiwan extending to the south of China Sea, it is very important fishery resources. The longline fishery operation began on spring in south of China Sea in spring, move to southwestern waters of Taiwan in the autumn. Yellowfin tuna catches poor performance in this year, the operation times just five months. During this fishing period, the longline catch per unit effort (CPUE) high of 0.6 fish/100 hooks in April, to a low of 0.2 fish /100 hooks in January. As the rises sea surface temperature (SST), yellowfin tuna catches with increased. The surface water temperature reaches 29 to 30℃, the fishing period is ending. The handline fishery operation began in September to November in southwestern waters of Taiwan. The handline CPUE high of 5.6/day in November, to a low of 1.3/ day in September. The yellowfin tuna catches significantly affected by water temperature changes, when the temperature rises to 26 ~ 28℃, the yellowfin tuna catches increases, showing yellowfin tuna this water temperature obvious preference.