A fuel performance code, COSMOS, was developed for an analysis of the thermal behavior and fission gas release of both mixed-oxide (MOX) and UO2 fuels up to high burnup. The models have been improved for the fuel thermal conductivity, the fission gas release, and the cladding corrosion and creep. In particular, the thermal conductivity and fission gas release models were restructured with due consideration for the inhomogeneity of the MOX fuel. These improvements enhanced COSMOS's precision for predicting the in-pile behavior of the MOX fuel. The COSMOS code also extends its applicability to the sophisticatedly instrumented fuel test in a research reactor. With the improved models, the recent in-pile test results were analyzed and compared with the code's prediction. The database consists of the instrumented MOX fuel test in a research reactor, the postirradiation examination results after an irradiation in a commercial reactor, and a preliminary instrumented test in the HANARO reactor. With the rigorously characterized fabrication data and irradiation information, the COSMOS code predicted the in-pile behaviors well, such as the fuel temperature, rod internal pressure, fission gas release, and cladding properties of MOX and UO2 fuels. The estimations by COSMOS also demonstrated its applicability to the instrumented irradiation test.