The conventional sensor MAC is unconcerned with data aggregation. Therefore, this paper proposes a new MAC level Data Aggregation (MDA). The goal is to reduce energy consumption in WSN (Wireless Sensor Network). The MDA has two functions that are discriminative to the conventional sensor MAC, in that it achieves efficient aggregation and the avoidance of Network beacon collisions. The MDA is designed considering the type of sensing data used in the network. Therefore, a node sleeps during its active period dependent on the sensing data since the node does not contain the sensor hardware itself. This means that energy consumption is reduced by preventing unnecessary transmission in the active time. Also, the MDA achieves effective aggregating, because it processes aggregation at the same range of active duration as that of the same sensing data type. Therefore, the energy-efficiency of the whole sensor network increases.