Femtocell access points (FAPs) has emerged as to improve the coverage and capacity of next generation network. However, dense femtocell deployments can cause severe interference. In this paper, a FAPs' cooperation approach is proposed to solve the interference problem. In particular, for uplink transmissions, the cooperative FAPs are allowed to form a group (i.e., coalition) to optimize their sum-rate by jointly decoding their received signals, and the FAPs' cooperation problem is formulated as a coalitional game in characteristic form with transferable utility based on multiple access channel (MAC). It is demonstrated that all the FAPs can form one group (i.e., grand coalition) to maximizes their sum-rate. Moreover, the core of the grand coalition is proved to be non-empty. Simulation results show that the proposed approach outperforms non-cooperative case in terms of the total system sum-rate and the average rate per link.