Two methodologies for predicting thermal relaxation time of tissue subjected to pulsed laser irradiation is introduced by the calculation the optical penetration depth and by the investigation of the temperature diffusion behavior. First approach is that both x-axial and y-axial thermal relaxation times are predicted and they are superposed to achieve the thermal relaxation time (${ au}_1$) for two-dimensional square tissue model. Another approach to achieve thermal relaxation time (${ au}_2$) is measuring the time required for local temperature drop until $e^{-1}$ of the maximum laser induced heating.