Abstract: The uncertainty of the proof strength at non-proportional extension of TC4 titanium alloy was evaluated. The evaluation results of the uncertainty of the proof strength at non-proportional extension with and without the extensometer were compared. The influence of the extensometer on the uncertainty of proof strength at non-proportional extension of TC4 titanium alloy was researched. The results show that the uncertainty of proof strength at non-proportional extension with the extensometer was evidently less than the uncertainty of proof strength at non-proportional extension without the extensometer.