Abstract: The polarization curves of titanium alloy TC4 in neutral water and 3%NaCl solution were measured and analyzed. Their cathodic and anodic Tafel slopes were calculated to be much greater than 0.118 V. The cathode process (hydrogen evolution) of titanium alloy in neutrol water was controlled by the reaction H⁺+H⁺+e=H₂⁺ which explained well the cathode Tafel slope was about 0.23 V. The nonstoichiometric oxide TiO_1+x(0.5<x<1) as an intermediate product took part in an electrochemical reaction was the controlled step in anodic process.