The present research aims to study the effects of normal stress level on the rock joint stiffness. In addition, the sample size effect was also taken into consideration. A total of 96 direct shear tests on samples with various sizes and different rock materials were conducted and extensively analyzed. Obtained results show as the normal stress applied increases and shear stiffness (Ks) and normal stiffness (Kn) of the joint increase. Travertine and granite have a similar dilative behavior at lower normal stress levels (lower than 1 MPa). However, when normal stress increases (4 MPa and higher), granite tends to retain its dilative behavior yet roughness in travertine has a significant impact on the joint behavior. An empirical natural logarithmic expression was proposed to describe the stiffness ratio of a rock joint. It was also understood that there is a correlation between the parameters of the logarithmic expression and the roughness of the joints. The empirical expression used to predict the stiffness ratio curves of different joint shows high conformity with the experimental results.