Energy hole is an inherent problem in wireless sensor networks, which results from heavier traffic loads of nodes nearer to the sink, and leads to significant network lifetime reduction. Cooperative communication can improve energy efficiency as well as balance energy consumption. In this paper, we use cooperative communication to mitigate energy hole and extend network lifetime. First, a cooperative transmission strategy is proposed, in which each node helps its relay node at the next hop to forward data through cooperative multi-input single-output (MISO) method. Second, to obtain optimal power allocation among cooperative nodes, we study both the optimization problem for network lifetime maximization (NLM) and that for energy consumption minimization (ECM). Numerical results show that our strategy effectively mitigates the energy hole problem and achieves much longer network lifetime than the conventional non-cooperative transmission strategy, in which nodes forward data hop by hop through single-input singleoutput (SISO) method. Moreover, it is demonstrated that the power allocation for NLM outperforms that for ECM in energy hole mitigation and network lifetime extension.