Vehicular ad-hoc networks (VANETs) is emerging as the most essential technology for securing traffic safety as well as enhancing traffic conditions. The confidentiality of messages transmitted in VANETs is of utmost priority, which has attracted extensive scholarly research. However, the security of the existing studied encryption schemes remains to be upgraded with a massy time overhead. Hence, we propose a lightweight encryption scheme for secure message transmission in VANETs with a private blockchain. Firstly, the encryption scheme we designed, called VMT, ensures the confidentiality and forward security of messages, while the system is sound and amnesic so that we are not entirely dependent on external servers. Meanwhile, the private blockchain with high privacy-preserving and flexible read/write access assures that merely permitted users to access the message. Furthermore, our scheme achieves excellent lightweight, thus suitable for VANETs with huge messages. Security analysis indicates that our scheme has firmly secured the transmission of messages in VANETs. Performance evaluation demonstrates that our scheme is lightweight and efficient, with encryption and decryption times of 7.00 ms and 7.67 ms, respectively.