Abstract: To study the effect of specimen size on fracture parameters in Mode I fracture issue, a group of geometrically similar sandstone specimens were tested by three-point bending. The result shows that as the size of specimen increases, apparent tensile strength decreases and the apparent fracture toughness and length of the process zone increase. These three factors are a function of size. Based on these test results, equivalent linear elastic fracture mechanics (ELEFM) was used to calculate fracture parameters such as the length of the process zone in rock material. The fracture mechanism of sandstone was identified to be between the perfectly plastic and perfectly brittle limits and the effect of size on quasi-brittle material was shown. To verify the correctness of ELEFM, the length of the process zone was obtained by digital image correlation (DIC) test and the experimental and theoretical results were found to fit well.