In a cluster-based wireless sensor network, to shorten the duration of a data aggregation process, wireless sensor nodes in different clusters may be required to perform concurrent transmissions. Inter-cluster interferences are usually resolved by using different channels among clusters. Since channel resource is limited, for large-scale networks, co-channel interferences are unavoidable. The problem becomes more severe when TDMA-based MAC protocols are employed. Such problem can be alleviated by controlling the transmission power levels of the concurrent users. In this paper, a transmission power control problem in wireless sensor networks is considered. The objective is to find optimum transmission power levels, which can obtain efficient trade-offs between energy consumption and the number of retransmissions. Different optimization schemes are investigated and analyzed using the concept of Pareto optimality.