Abstract: Silicon and silicon coating system are extensively used in the engineering of micro electronic and mechanical systems (MEMS). Nano indentation is usually used to measure the material hardness and assess its tribological properties. Thus nano indentation test was carried out to measure the hardness of silicon crystals and investigate the feature of indentation load versus depth the curves. The stress field distributions of the silicon crystal during the loading and unloading process of indentation test were analyzed. The Mises stress field and stress fields in six directions indicate that the failure of thin wafer of silicon crystals during nano indentation test is attributed to the spalling under shearing and normal stress.