A method for broadcasting a message in a vehicular ad hoc network and a vehicle mounted communication system are provided. The method may include: a first communication device mounted on a first vehicle receiving a message from a second vehicle; identifying a first number of vehicles which are within a communication range of the first communication device; calculating a first count-down time based on the first number, where the greater the first number the shorter the first count-down time; and if the first communication device does not receive the message again before the first count-down time ends, broadcasting the message when the first count-down time ends. The efficiency and reliability of message broadcasting may be improved.