Under compression, gathering data related to the post-failure part of the stress-strain curve requires stiff servo-controlled testing systems. In unconfined conditions, data related to the post-peak region of the intact rock parameters are not common as pre-peak and peak state parameters of stress-strain behavior. For problems involving rock in the failed state around structures, proper choice of plastic constitutive laws and post-failure parameters is important for the modeling of the failed state. The aim is to relate commonly used intact rock parameters of pre-failure (tangent modulus E (i) and secant modulus E (s)) and peak strength (sigma (ci)) states to parameters of the post-failure state under unconfined compression. Post-failure parameters are the drop modulus (D (pf)), representing the slope of the falling portion in brittle state, residual strength (sigma (cr)), and dilatancy angle (psi A degrees). Complete stress-strain curves were generated for various intact rock of different origin. Seventy-three post-failure tests were conducted. Samples included in the testing program were chosen to represent rocks of different origin. Specimens of granite, rhyodacite, dunite, quartzite series, glauberite, argillite, marl, and lignite were used in the tests. The results from the pre-failure and peak state testing parts were processed and compared to the post-failure stress-strain parameters. For the estimation of post-failure parameters in terms of the pre-peak and peak states, the functional relations were assessed. It was found that the drop modulus D (pf) increases with rock strength sigma (ci), following a power function with an approximate power of two. With an exponential trend, the D (pf)/E (s) ratio increases with decreasing E (i)/sigma (ci) ratio. Relations estimating the residual strength and dilatancy from the pre-peak and peak state parameters are in logarithmic and exponential functional forms, respectively.