The frost shattering of rocks was traditionally explained by the volume expansion of water in pores of rocks. However, the frost heave mechanism with coupled heat and moisture in a porous rock during freezing was proposed. According to this new concept, the water in pores of rocks freezes at below zero degrees due to its lower level of chemical potential. Thus a fairly large potential gradient in rocks is developed, and then water migrates along the gradient to freezing front. To verify this concept, the freezing point depression of water in pores was measured. Both temperature in rocks and ultrasonic wave propagation velocity through rocks were measured during freezing and thawing. At the time of the freezing of the water, temperature and velocity change simultaneously. By the time a discontinuity of velocity change is detected, the freezing temperatures of an unsaturated porous rock are determined. Based upon the experimental results, the empirical equation for freezing point depression as a function of the degree of water saturation is obtained as follows: DELTAT = A + B . log S where DELTAT: freezing point depression (C°) S: degree of water saturation in pores (%) A, B: constants. Using this equation, the chemical potential of water in pores of rocks m ight be estimated.