Wireless sensor networks (WSNs) are expected to play an essential role for our society in the near future. In this paper, we propose a data adaptive transmission mechanism, DAT in short, for clustering-based wireless sensor networks. Sensor nodes do not always have data to send all the time. The data obtained at a time by the sensor nodes are expected to be transmitted as a whole to the cluster-head as soon as possible. The sensor nodes will send request message to cluster-head for data transmission slots to transmit its data. The cluster-head assign appropriate slots adaptively to the sensor nodes according to current requests of all the cluster members. Then, the nodes transmit their data during their allocated transmission slot. At last, the cluster-head aggregate the data received from sensor nodes and send the effective information to the base station. Simulation experiments show that the DAT mechanism outperforms other clustering-based protocols in wireless sensor networks.