Taiwan Coastal-Offshore Fisheries, for lack of effective fisheries conservation, which are in face of random competition and excessive exploitation of resources under vicious circle, have made resources exhausted in the sea area near Taiwan and have diminished catch of fish. CPUE (Catch per Unit of Effort) was decreased year after year because of excessive competition, so that it caused income of the fishing population to get in a crisis. The fact that we know, in order to solve effectively the operating difficult position in Coastal-Offshore Fisheries, is that the government adopt an phase-in on elimination and construction system in connection with the fishing boat and some measures of vessels reduction, for reducing amount of fishing boat and fishing effort, to promote the vitality and the competitive ability of production in Coastal-Offshore Fisheries. The basic theory of vessels reduction comes from the following backgrounds: 1. A viewpoint of economy in bio-ecology: when experts decide the whole policy according to this mode, the goal to determine fishing effort can be selected among divers fishing effort which was made by laissez-faire economy of completely free competition, Maximum Sustainable Yield (MSY), and Maximum Economic Yield (MEY) . 2. A viewpoint of welfare economics: to achieve the goal of welfare economics with the thoughts of whole people, the intersection of marginal cost and average income is the most proper that we can obtain from the maximum sum of producers surplus and consumers surplus as fishing effort. 3. A viewpoint of external diseconomies: when marginal value of people is equal to marginal cost of fishing population, this point will be the most proper for fishing effort. The purpose of study in this thesis is to use a method of analysis exploring the correctness of vessels reduction program that the fishery authorities in our government execute, and to look over its effect to find out whether it has achieved the goal we expected. We have discovered the result through cost-benefit analysis of actual examples, of which the vessels reduction program to purchase the old fishing boats for lack of a complete set of other measures which based on fishing effort and fishery management in the theory of vessels reduction , will cause the measures to fail to improve remarkably these two important goals¢wto decrease the fishing pressure, and to promote the fishing gains¢win an expectable time. This study propose that the vessels reduction program could consider cutting down fishing effort with other methods for pursuing the eternal development in Coastal-Offshore Fisheries, such as to rest fishing and to carry out the measures of a complete set in a ration system, and that the effect, except the proper fishing effort made by the consideration in basic theory of vessels reduction , will be better. Furthermore, an object of vessels reduction should not only be limited to the old fishing boats over 15 years of age, but also take the ones as object which remain working at sea and have reached over 2 years of age in case of uneasy or bad operation. As before, it will be able to attain the aim of decreasing competition in fishing circles.