AbstractThe COMETHE II computer program has been formulated to predict the thermal and mechanical behavior of fuel pins during their irradiation life. It calculates, in particular, the temperature distribution, the radial and axial fuel swelling and expansion, the fission gas release, and the stresses and strains in the cladding.The program involves many models for this purpose. Some of these have been separately tested and calibrated with respect to available experimental results in the literature. Calibration of the whole program is also being currently performed. COMETHE II results are compared with experimental data for different burnups and thermal ratings. The agreement obtained with the experiments is rather good, the theoretical data lying generally in the margin of error of the experimental data.The capabilities of the COMETHE II program are illustrated by a parametric study of the gap width influence on the maximum center temperature and on the strain of the sheath. This example shows that the COMETHE II program is a useful tool for the design of fuel pins for fast or thermal power reactors.