Abstract: The crack length of compact tensile specimens of metal materials was measured using the flexibility method， and the sources of uncertainty during the measurement process were analyzed. The uncertainty of this method was evaluated. The results show that the main factors affecting uncertainty during the measurement process were sample size and unloading flexibility. The uncertainty report of the crack length measurement results of the sample showed that the crack length of the sample was 22.73 mm， the expansion uncertainty was 0.10 mm， and the inclusion factor was 2.