In this paper, we propose two certificate mechanisms for preventing the Sybil attack in a vehicular ad hoc network (VANET): the timestamp series approach and the temporary certificate approach. We focus on an early-stage VANET when the number of smart vehicles is only a small fraction of the vehicles on the road and the only infrastructure components available are the roadside units (RSUs). Our approach does not require a dedicated vehicular public key infrastructure to certify individual vehicles but RSUs are the only components issuing certificates. The vehicles can obtain certificates by simply driving by RSUs, without the need to pre-register at a certificate authority. The timestamp series approach exploits the fact that because of the variance of the movement patterns of the vehicles, it is extremely rare that the two vehicles pass by a series of RSUs at exactly the same time points. The vehicles obtain a series of certificates signed by the RSUs, which certify their passing by at the RSU at a certain time point. By exploiting the spatial and temporal correlation between vehicles and RSUs, we can detect the Sybil attack by checking the similarity of timestamp series. In the temporary certificate-based approach, an RSU issues temporary certificates valid only in a particular area for a limited time. To guarantee that each vehicle is assigned only a single certificate, at the issuance of the first certificate, it is required that the RSU physically authenticate the vehicle. When driving by the subsequent RSUs, however, the certificate can be updated in a chained manner. By guaranteeing that each vehicle is issued a single certificate in a single area, the Sybil attack is prevented. We provide mathematical analysis and simulation for the timestamp series approach. The simulation shows that it works with a small false-positive rate in simple roadway architecture. Copyright © 2013 John Wiley & Sons, Ltd.