Recent advances in high-speed networks have made micro-electro-mechanical systems (MEMS) find some niche applications in tunable optical devices. Indium phosphide (InP)-based MEMS have an inherent advantage of being direct band gap as compared to silicon and can thus be used in MEMS structures with light emission/detection capability. In this paper, we report an in-depth study using nanoindentation to determine the mechanical properties of InP free-standing structures that could be incorporated in optical MEMS. The fabrication process for InP-based cantilever beams and membranes is also reported. Young's modulus of the material is determined from both loading and unloading of the InP cantilever beam through a bending test. We also discuss the deformation behaviour of the InP cantilever beam. Indentation on an InP substrate was conducted using a spherical indenter of known radius, in addition to the conventional Berkovich tip. The results were compared with ideal analytical methods. Experiments were also carried out to determine Young's modulus and hardness using continuous stiffness mode (CSM) tests. In addition, as a benchmark, experiments on silicon and sapphire substrates are also discussed. The results show good agreement in the mechanical properties obtained through different experiments.