A simple model for estimating temperature distribution within the frozen region during cryo ablation was tested for accuracy. Freezing experiments were conducted in both ex vivo and in vivo porcine livers. Temperature was measured during freezing using a fiber-optic temperature sensor. Three-dimensional MR images were obtained at the end of each freezing cycle. From the MR image volumes, three-dimensional temperature maps were calculated numerically using a simplified bio-heat model. Estimated temperatures were compared to measured temperatures. The median difference between measured and estimated temperature was 3.03 degrees C. The median distance from a sensor element to the closest point on a isotherm surface with the corresponding estimated temperature was 0.70 mm. The accuracy of this model is acceptable. Temperature maps as outlined here may be used for monitoring of cryotherapy in order to increase clinical effectiveness.