In laser assisted microprocesing, such as laser induced diffusion, the substrate is irradiated by a focused laser beam. And a high temperature region is formed on the substrate surface. To acquire the desired temperature distribution, the temperature rise in a semiconductor substrate induced by 10.6 μm focused continuous wave (CW) CO2 laser beam has been investigated numerically. The temperature-dependent absorption coefficients of the substrate material are incorporated in the calculation model. The relations between the temperature rise and parameters such as the preheating temperature, the power of the laser beam and the beam width have been obtained. It is shown that when the substrate is in room temperature before the irradiation and the diameter of the focused laser beam is smaller than 100 μm, the highest stable temperature on the substrate can not exceed 600 K. It is also shown that increasing the preheating temperature can reduce the size of high temperature region, when the induced temperature rise is kept to meet the requirement. Under the condition that the temperature distribution meets the experimental requirement, the laser spot size and the power of the incident laser beam should be adopted as large as possible in order to make the control of the temperature distribution easier.