Abstract: The distributed TDMA approach for vehicular ad hoc network (VANET) does not take advantage of idle slots, failing to effectively utilize radio resources, and the approach is not free from packet dropping due to a poor channel condition. Cooperative communication, on the other hand, has drawn significant attention from both academia and industry in recent years, since it can be effective in mitigating wireless channel impairments by utilizing the broadcast nature of the wireless channel. In the paper, a cooperative scheme for medium access control (MAC), referred to as cooperative distributed TDMA (Co-DTDMA) is presented for VANET. In Co-DTDMA, neighboring nodes utilize unreserved slots for cooperatively retransmitting a packet which has failed to reach the destination node owing to a poor channel condition. Different from traditional cooperative approaches, all Co-DTDMA operations, such as synchronization among nodes, reserving a time slot, cooperation decision and cooperative transmission are done in a fully distributed manner, which makes it suitable for VANET. In addition, cooperative transmission is conducted in unreserved slots, without interrupting the normal transmission. Both theoretical analysis and experimental results demonstrate that the proposed scheme significantly increases the probability of successful packet transmission and decreases the delay of packet transmission in various network parameters.