Tantalum was prepared by solid oxygen-ion membrane (SOM) process from Ta2O5 directly in molten flux CaCl2 system. The micro-structural features and the porosities of the Ta2O5 pellets prepared at different pressures and sintered at different temperatures were investigated, and then the effect laws of the cathode microstructure on product morphology and oxygen contents were obtained. The results show that the particle size of cathode pellets and porosities are importance factors for the electrochemical reduction process. The porosities of the Ta2O5 pellets decrease remarkably as the pressure and sintering temperature increase. High porosity can enhance the rate of deoxidization and obtain bigger particle products; contrarily, a compact outer layer of metal tantalum is formed. When the Ta2O5 pellets are pressed at 4 MPa and sintered at 1150 oC for 2 h (or pressed at 6 MPa and sintered at 1100 oC for 2 h), electrolysis reaction activity is good and electrolytic products have low oxygen content.