We fabricate a high Q photonic crystal cavity on the top of SOI (silicon on insulator) with EBL (electron beam lithography) and ICP (inductively coupled plasma). The value of Q can reach 7 104. It provides basic condition for the following experiments, for example for the study of interaction between light and substance. The high Q cavity also provides good circumstance for the quantum information. The theoretical result of the value of Q is 1.2105 from FDTD (finite difference time domain) simulation.