This paper analyzes the equilibrium size and optimal size of research joint venture (RJV) under open membership rule and restricted entry rule. Three assumptions are made here: there is only one RJV; research is for the objective of reducing cost: and there exist intra-industry spill-over effects of innovation result. In the conventional closed market case, under open membership rule the coalition size is far greater than the socially optimal size. However, under the restricted entry rule the coalition size is closer to the socially optimal size. So, the later rule is consistent with the spirit of the Fair Trade Law. But in open market case, under open membership rule the coalition size may come closer to the socially optimal size. With restricted entry rule the coalition size may be far less than the socially optimal size, and the restricted entry rule is not consistent with the Fair Trade Law.