 Abstract-The utilization of IoT technology increases uninterrupted. The sensors in IoT environments collect great amounts of data and give them to the service providers. This generous content of user's individual data for IoT applications introduces new security challenges. Indeed privacy preserving is a very significant issue for IoT applications. In this paper, we propose a protocol for privacy preserving in IoT environments using blockchain technology and smart contract concept. Blockchain provides a condition that non-trusting members can correlate with each other without a trusted intermediary. Smart contract is a computer code that executes the conditions of a contract itself. Smart contract helps us to exchange our valuable things in a transparent way without any need to a middleman. The key issue in this work is a secure transmission of the shared key between a sender and a receiver using smart contract and blockchain technology in IoT environments. The secure exchange of shared key can result privacy preserving. We have implemented a prototype of the proposed protocol using pyethereum, and evaluate its security via AVISPA tool.