A laboratory investigation is undertaken to examine the peak dilation angle of basalt and marble using a servo-controlled tri-axial apparatus. The rocks are tested at different confining stresses of 2 MPa–28 MPa. The peak dilation angle is determined by calculating the ratio of plastic volumetric strain and plastic shear strain. The peak dilation angle is decreased with an increase in the ratio of σ3/σc, and the logarithmic function is fitted with the data. The findings suggest that basalt exhibits a higher peak dilation angle in comparison to marble rock. The intersection point between elastic and plastic axial strain is established for basalt and marble, demonstrating the threshold at which plastic deformation dominates over elastic behavior. This information offers a valuable understanding of the characteristics of elasticity and plasticity observed in tri-axial rock tests. This research will augment the current database on the examination of dilation angles in the domain of rock mechanics.