Vehicular Ad Hoc Network (VANET) is an application of Mobile Ad Hoc (MANET) for road traffic. VANET has the characteristics of high moving speed, frequent changing topology, and different node densities. In this dissertation, according to the characteristics of VANET, it is aimed to improve the performance of MAC layer and provide reliable and real-time communications between vehicles. Modeling and analysis of periodic broadcast, modeling and analysis of broadcast in control channel, dynamic contention window scheme, MAC delay based clustering algorithm and cluster based time division multiple access are studied in detail.The main research contributions are as follows.(1) Modeling of periodic broadcast and dynamic contention window scheme in VANETA 1-D Markov model is proposed to analyze the performance of periodic broadcast in VANET. In this model, a new idle state is introduced under non-saturated condition when there is no message to send in the buffer of a node. The freezing of backoff time counter in backoff process is also considered in this model and a discrete time D/M/l queue is established to model the buffer of each node. Theoretical analysis show when vehicle density increases the performance of periodic broadcast decreases accordingly. Dynamic contention window scheme is proposed according to the changing of node density. Simulation results show the collision probability of DCW scheme is lower than that of fixed-contention window broadcast in IEEE 802.11 p. Simulation results also verify the accuracy of the Markov model.(2) Modeling of priority access to control channel in VANETAccording to the characteristics of messages with different priorities accessing to control channel, discrete time queue D/M/1 and M/M/1 are proposed to model periodic messages and emergent messages, respectively. Priority analysis is added to this model which is based on the previous 1-D Markov model. Packet collision probability, access delay of periodic message and emergent message and performance of DCW scheme in control channel are analyzed. Simulation results show DCW scheme could reduce the collision probability, which is suitable for control channel with different priority messages.(3) MAC delay based clustering algorithmThe performance of MAC protocol decreases in high vehicle density scenario. A MAC delay based adaptive (MDBA) clustering algorithm is proposed. Under the restriction of MAC delay, speed, acceleration, position and destination are considered in cluster head election algorithm. Cluster maintenance algorithm adaptively adjusts cluster according to changes of network topology. Traffic simulation software VISSIM is used to verify the proposed algorithm, and simulation results show that MDBA clustering algorithm performs better than classic clustering algorithms in wireless sensor network and mobile Ad Hoc network, which is more suitable for VANET.(4) Cluster based time division multiple access in VANETIn dense vehicle scenario, time slots are not available for all nodes in TDMA based access scheme. To solve this problem, a cluster based time division multiple access method is proposed. In this method, position based segment time slot allocation is employed. Cluster members are divided into two teams and allocated to two time segments. Moreover, slot reservation scheme is proposed to reduce collision and transmit emergent messages as soon as possible which lowers the access delay of emergent messages. Simulation results show the proposed method could reduce the collision probability and access delay of emergent messages in small traffic flow, compared with traditional TDMA scheme in VANET.