Abstract: An automatic digital image processing technique is presented for whole-surface measurement of specimens' deformations during triaxial tests. This method incorporates digital imaging techniques and computer-aided analysis to assess the deformation throughout a triaxial test. A detailed description of hardware as well as other equipment is provided. Regulated power supply along with a darkened chamber is adopted to ensure the stability of the lighting environment. Data analysis methodology including distortion correction, pixel equivalent normalization, strain field calculation and data splicing is provided. The calculation of strain is based on the assumption that in each neighborhood of time and space, specimen only deforms along radial. Finally, the advantage of this whole-surface deformation measurement in analyzing deformation localization for triaxial tests is discussed through results obtained from drained triaxial tests on saturated sand.