Internet of Things (IoT) has been experiencing exponential growth in recent years, but still faces many serious challenges. The distributed ledger technology (DLT), e.g., Blockchain, not only appears to be promising to address these technical challenges, but also brings tremendous opportunities for new application and business models. However, the convergence of IoT and DLT is yet a goal far beyond our reach today. Among many problems that have not been sufficiently understood, a fundamental one is how to design appropriate consensus mechanisms for DLT applied to IoT, which is the theme of this paper. We first discuss the potential benefits of applying DLT to IoT, and identify major challenges posed to DLT by IoT. Then we make a survey of existing DLT consensus mechanisms, to summarize major principles and discuss their pros and cons when applied in IoT.