In the TRISO (tristructural isotropic) coated fuel particle used in the High Temperature Reactor, the most important layer is a silicon carbide layer which acts as a pressure vessel. In this study, we have focused our study on the investigation of the Young's modulus, hardness, residual stress, and fracture toughness of the SiC layer. Moreover, microstructures and impurities in silicon carbide were characterised and then related to both Young's modulus and hardness of the SiC layer. Both nanoindentation and micro-indentation were used to determine Young's modulus and hardness of the SiC. Raman spectroscopy, X-ray diffraction, and scanning electron microscopy techniques were used to examine impurities, phases and microstructure of silicon carbide layers, respectively. Young's modulus was measured at different positions of a polished surface of the SiC with different CVD growth and crystal orientations. With help from the finite element modelling, it has been found that Young's modulus of the SiC is dependent on the grain orientation of the SiC. Mechanical properties of silicon carbide are affected by the presence of excess silicon, excess carbon, stacking faults, texture, grain size, property of grain boundary. The effect of these factors on Young's modulus and hardness, are investigated with the orthogonal analysis. The analysis concludes that the most important factor on Young's modulus is texture while the most significant factor on hardness is grain boundary. Grain size is secondarily important factor to affect hardness. Stacking faults and impurities almost have no influence on Young's modulus and hardness. The residual stress in the silicon carbide layer was measured based on the peak shift in Raman spectra of the SiC and is in a range of 150-300 MPa. Fracture resistance in the radial direction of the SiC layer is larger than those in the circumferential direction. The difference is controlled by the layer-like structure of the SiC coating.