Abstract: The plane-strain fracture toughness KIC test model was elaborated, and the mechanical of KIC validity criterion in GB/T 4161-2007 was defined. On that basis, the effect of pre-crack stress intensity factor and tensile speed on KIC test results was analyzed. It was pointed out that increasing specimen thickness was an effective way to improve the possibility of getting valid KIC test results. In order to prove this viewpoint, the KIC testing results of TC4 and TA15 titanium alloys were illustrated.