The Internet of Things (IoT) provides an infrastructure enabling advanced functions based on interoperable information and communication technologies by interconnecting physical and virtual things. IoT has gained more and more attention from both academy and industry. However, the design, maintenance, and governance of IoT are facing challenges because of an enormous amount of heterogeneous devices involved. IoT requires strengthening its trustable features. The blockchains are immutable distributed ledger systems supporting trustable features including security, scalability, privacy, safety, and connectivity usually without a central authority. Due to the complexity of IoT system any current blockchain cannot satisfy all demands of heterogeneous devices, networks, and higher level functionalities. This paper proposes a novel reference architecture combines Blockchain Federation with IoT to make IoT capable of being trusted. The architecture enables trustable features by integrating blockchains with different IoT tiers and layers. Proof of concept system is implemented to demonstrate the correctness and feasibility of the architecture. Furthermore, the future directions of Blockchain Federation and IoT evolution are discussed.