Taiwanese offshore tuna longline fishery is that the fishing vessel is middle and small scale with the tonnage class between CT-2 and CT-4 and supplied on domestic ports. The catch and effort data used in this study was obtanined by inquiring the skipper of the returned vessels at Nanfanao. There are 613 course data was colleted in tuna fishing season, from April to July, during the period 2004~07. The CPUE is the effort divided by catch. The effort is the number of operating days, and catch is the sum of the catch in number and/or in weight of yellowfin, bigeye, albacore and bluefin tuna. By applying the GLM procedure in the effort standardization, it shows the model effect was always signigicant by which the CPUE in number or in weight was used, respectively.However only the CPUE in weight in model can result of significant differences of the fishing power among three tonnage class vessels. If the fishing power of CT-4 was set to 1, the relative fishing powers of CT-2 and CT-3 would be 1.49 and 1.48. Not only have the similar fishing powers between CT-2 and CT-3, the catch compositions between two classes are more alike.