Compared with traditional insurance schemes, usage-based insurance (UBI) for vehicles is more economic and accurate for drivers since its insurance premium calculation depends on how vehicles are driven. However, UBI requires sensitive driving data to determine insurance premiums, and this could result in serious privacy breach for drivers. Meanwhile, existing UBI solutions rely on a centralized entity (i.e., the insurance company) to manage insurances. In this article, we design a decentralized and privacy-preserving UBI scheme, called DUBI, based on the blockchain technology and zero-knowledge proof. In our scheme, a smart contract running over the blockchain serves as a “decentralized” insurance company, while drivers continuously upload their committed driving data to the blockchain. Periodically, the driver submits accumulated driving statistics with a zero-knowledge proof to the smart contract, which verifies the proof and calculates the insurance premium from the submitted statistics. We formulate an ideal functionality for DUBI under the universal composability framework, and then provide a formal security proof for DUBI. Furthermore, we give in-depth analysis and performance evaluation for DUBI with an implementation based on Ethereum. It shows that DUBI is highly efficient in processing UBI insurances in both storage and computation: DUBI is about seven times more efficient than existing schemes in storage, and proof generation and verification take only 7 and 30 ms, respectively.