In wireless sensor networks (WSNs), the strategies of periodical sleeping and contending for using the channel are efficient in power consumption and channel utilization. However, the situations of contention and sleep arise another power consumption problem of overhearing the controls packets and increasing transmission latency. In this paper, we propose the design of nodes grouping and transmission pipelining to improve the power consumption and transmission delay. In nodes grouping design, there are several groups in the WSNs and each sensor node is initially set to belong to one of these groups where each group has different contention time and transmission time. In contrast to the situation that all nodes hear the control packets in the contention period, nodes grouping can reduce the number of nodes that overhearing the control packets in the same time to save the power consumption. To improve the transmission delay, we propose the design of transmission pipelining that makes the group number of the nodes on the path to be continuous. Therefore, the sensor node can transmit data to the sink node pipelining. From the simulation results, when the number of groups is 2, the power consumption of transmitting a byte (mJ/byte) and the transmission delay of our proposed design perform better than SMAC by about 50%. When the number of groups is 4, although the transmission delay shows only a little better than SMAC, the power consumption of transmitting a byte of our proposed design has much better than SMAC by 75%.