Although optical extensometers based on Digital Image Correlation (DIC) method have widespread applications in strain measurement, there is a lack of systematic research on the strain measurement accuracy. For the uniaxial tensile test, testing equipment including the traditional 2D-DIC optical extensometer, three-dimensional digital image correlation (3D-DIC) optical extensometer, 2D-DIC optical extensometer equipped with dual-reflector imaging capability, and strain gauge were employed to measure the strain of stainless steel simultaneously. The results of axial and transverse strain measurement using different optical extensometers were compared, and the corresponding sources of measurement error were analyzed. The experimental results revealed that the traditional 2D-DIC optical extensometer yields low measurement accuracy because of the presence of out-of-plane motion. The 2D-DIC optical extensometer equipped with dual-reflector imaging capability can eliminate the effect of out-of-plane rigid body displacement and thus, ensure higher measurement accuracy, especially for transverse strain measurement, compared to other optical extensometers. Notably, the strain measurement results obtained using the 2D-DIC optical extensometer equipped with dual-reflector imaging capability were in good agreement with those obtained using a strain gauge. The measurement accuracy of transverse strain using 3D-DIC optical extensometer is apparently lower than that of axial strain.