Abstract: A digital image processing technique for measuring the specimen deformation in triaxial test is presented. The image is gained by a CCD and the digital signals of the specimen image are input into computer with a image interface card. Based on identification of the image edge according to the change of brightness, the shape and the deformation of the specimen are automatically determined in real time. This technique can be efficiently and conveniently applied to traditional triaxial test apparatus. It can be used to measure the deformation of saturated and unsaturated soil specimens. Accuracy of the measurement is analyzed. Reasons that can cause the error of the measurement are also discussed in this paper. Measured results of soil specimen deformation are given for comparison between traditional method and the technique presented in this paper.