Joint transmission (JT) is one of multi-cell coordination schemes for efficient support of cell-edge users. In order to achieve high capacity based on JT, it is required to appropriately allocate transmit power such that it does not incur interference to other cells. In this paper, we propose a power allocation algorithm for JT based on multilateral bargaining model. The discount factor is defined by explicitly considering the channel quality, so that the base stations with high discount factor can have more transmit power. This leads to high overall channel capacity. The simulation results show that the proposed algorithm enables base stations to allocate transmit power and to achieve higher capacity of a cell-edge user.