Line-of-sight (LOS) blockage is a crucial problem in millimeter-wave vehicle-to-vehicle communication due to its severe penetration loss and high vehicle mobility. In order to overcome the LOS blockage problem, in this letter, we propose using neighbor vehicles as relays to forward the blocked traffic flows. Specifically, a traffic-aware relay vehicle selection is investigated. First, analytic hierarchy process (AHP) is adopted to handle the situation that different traffic types have different preferences on the performances of rate, delay and data dropping ratio. Second, we introduce a coalitional game (CG) to evaluate the relative performances of the relay vehicles. Finally, by combining the results of AHP and CG, a heuristic relay selection scheme is devised by selecting the relay vehicle with the best rationality degree. Simulation results show that the proposed scheme can adapt to the requirements of different traffic types and improve the performances of rate, delay and data dropping ratio when LOS blockage occurs.