This paper presents a new MAC protocol to achieve energy efficiency and low transmission latency for the static wireless sensor networks. The proposed new protocol is called A-MAC, an Alternative MAC, which assigns modified alternative wakeup schedules to different sensor nodes to minimize the probability of transmission collision. In A-MAC, after all sensor nodes are deployed, the base station will start an initial process to set a height for each node. When the height of each node (= the node's hop counts to the BS) is decided, the active interval of nodes whose height difference = 1 will be set to be continuous to reduce transmission latency, whereas the active interval of nodes with the same height will stagger to avoid collision (i.e., to reduce the probability of simultaneous transmission). Operating by such alternative wakeup schedules, the new protocol is able to smooth data transmission and meanwhile conserve energy consumption for the energyconstrained wireless sensor networks.