The ubiquitous deployment of Internet of Things (IoT) devices enhances connectivity and communication, and benefits almost every aspect of our lives from manufacturing to retail to smart homes. However, low levels of security protection in these devices due to their limited resources open opportunities for malicious users. An IoT forensics system collecting, processing, analyzing and reporting evidence of attack is required to mitigate the IoT security issues. Although such system has been studied over the past decade and solutions such as cloud-based IoT forensic were proposed, limitation still exist. In this paper, leveraging on the blockchain technology, we propose a per-missioned blockchain-based IoT forensics framework to enhance the integrity, authenticity and non-repudiation properties for the collected evidence. We formally define the system architecture, provide framework details, and propose a cryptographic-based approach to mitigate identity privacy concern.