In this paper, we introduce a message physical layer assisted message authentication (PAA) framework under public key infrastructure (PKI) in vehicular communication networks. The proposed framework takes advantage of temporal and spatial uniqueness in physical layer channel responses for each transmission pair, in which a trust between two vehicles can be maintained by comparing the current estimated channel response and the previous estimated channel response. We will show that the proposed message authentication framework can achieve high efficiency and minimal authentication delay without compromising the security requirements, which is further verified through both analysis and simulation. In this paper, we introduce a message physical layer assisted message authentication (PAA) framework under public key infrastructure (PKI) in vehicular communication networks. The proposed framework takes advantage of temporal and spatial uniqueness in physical layer channel responses for each transmission pair, in which a trust between two vehicles can be maintained by comparing the current estimated channel response and the previous estimated channel response. which will further improved compromise the security requirements, which is further verified through both analysis and simulation.