3D finite difference computerized model was developed to simulate the thermal behavior of biological tissue irradiated with a CO2 laser beam. The model uses standard thermal processes and in addition takes into consideration water evaporation and the corresponding changes in the thermal properties of the tissue. The results contain a temperature and biological damage map of the irradiated tissue. Experimental results of test made on Lucite irradiated with CO2 laser fit very well the theoretical predictions. The model was then used for simulations of tissue welding and the results indicate that an improved method could be used for CO2 laser welding.