A system and method for measuring biomechanical properties of tissues without external excitation are capable of measuring and quantifying these parameters of tissues in situ and in vivo. The system and method preferably utilize a phase-sensitive optical coherence tomography (OCT) system for measuring the displacement caused by the intrinsic heartbeat. The method allows noninvasive and nondestructive quantification of tissue mechanical properties. Preferably, the method is used to detect tissue stiffness and to evaluate its stiffness due to intrinsic pulsatile motion from the heartbeat. This noninvasive method can evaluate the biomechanical properties of the tissues in vivo for detecting the onset and progression of degenerative or other diseases and evaluating the efficacy of therapies.