The performance of WSNs is adversely affected by the radio irregularity and fading effect. Cooperative transmission has been proven to be an effective way to combat the impacts of fading by obtaining diversity gains and therefore reduces the transmit energy. Meanwhile, some sensor nodes have heavier burden than the others and the energy imbalance problem remains harmful to the system lifetime. How to efficiently incorporate cooperative transmission into WSNs and balance energy are the subjects of this thesis. In our research, we proposed an energy-balanced parameter-adaptable cooperative protocol (EBPACP) for cluster-based WSNs. Since the design of WSNs is highly dependent on application scenarios, we analyzed the effects of the system parameters and found a unified criterion to summarize the effects. With this knowledge, a preferred scheme of cooperative transmission is chosen to reduce energy consumption. How to form clusters, build up cooperative relationships and transmit data to the BS are explored in detail in the thesis. We use the idea of weighted distance to adjust the size of the cluster. Thus energy consumption is balanced by adjusting energy dissipated inside and outside of the clusters. Sensor nodes consuming higher energy in outside-cluster communication form smaller clusters. In this thesis, EBPACP is mainly discussed in single-hop systems and it can be extended to multi-hop systems. Simulation results have shown that the proposed EBPACP provides good system performance in terms of energy efficiency and energy balance.