Small punch tests were conducted for carbon steel specimens prepared with and without cold working. A digital image correlation technique was applied to measure displacement. Finite element analyses using the true stress-strain curves including post-necking strain could reproduce the load-displacement (LD) curve. Use of the nominal stress-strain curve obtained by the conventional tensile test was not enough to simulate the maximum load. It was found that the maximum load did not depend on the ultimate strength. Therefore, the ultimate strength prediction using the maximum load in the LD curve may not be valid for the general case. On the other hand, the yield strength correlated well with the deflection point in the LD curve. A new equation was proposed to estimate the yield strength that takes into consideration the influence of the specimen thickness.