Abstract: The Internet of Things (IoT) is poised to transform people’s lives and unleash enormous economic benefit. However, inadequate data security and trust of current IoT are seriously limiting its adoption. Blockchain, a distributed and tamper-resistant ledger, has the potential to resolve the myriad of security, data integrity and reliability concerns in IoT. Integrating blockchain technology into IoT has many challenges, including massive IoT devices, limited computing power, low bandwidth and error-prone radio links, which can have significant impact on the consistency and capacity of the blockchain based IoT. After survey existing theories on consistency and capacity in distributed networks, blockchain-based IoT security architectures is discussed, and future directions in optimally leveraging consistency and capacity of blockchain for IoT applications is pointed out with a review of relevant theories.