<p>To investigate the influence of porosity on mode I fracture toughness (<i>K</i>_IC) of rock-like material, lacustrine silty clay from the Chengmenshan Copper Mine was selected as the material. Porosity of semi-circular disk specimens was quantitatively controlled using consolidation tests, and three-point bending tests measured <i>K</i>_IC values. The results indicate that porosity has a significant effect on the fracture behavior and mode I fracture toughness of the rock-like samples. Lower porosity increases the likelihood of cracks propagating linearly along the centerline and results in a smoother fracture surface. As porosity increases, crack propagation becomes more deflected and fractures occur in a curved pattern. The deformation of the samples was observed using digital image correlation techniques, and the results indicated that the displacement on both sides increased as the crack propagated with a clear symmetric pattern. The opposite directions of the displacement showed that the crack could be characterized by distinct tensile failure. The mode I fracture toughness decreased with increasing porosity, and the rate of decrease accelerated when the porosity was larger than 38.3%. Finally, a correlation formula between porosity and fracture toughness for rock-like materials was established. The findings could contribute to the study on factors affecting the fracture toughness of rock-like materials.</p>