The thermal blockage and thermal round flow in HFCVD system for CVD diamond growth will lead to un-stability of product quality. Finite element method has been used to simulate the gas flow field around the cutting tool substrate within a HFCVD diamond reactor. Experiments have been done to prove the simulation results. Excellent agreement between simulation and experiment was obtained by depositing of CVD diamond coated cutting tool. The thermal blockage and thermal round flow in HFCVD system decrease by using a hollow substrate holder. High quality CVD diamond coating can be obtained using a hollow substrate holder.