The effect of operating cycle and fuel burnup on the isotopic composition and decay characteristics of irradiated nuclear fuel has been investigated using a standard computer code, KORIGEN. The parameters studied include isotopic compositions of actinides; activities due to the actinides, fission products, and light elements; decay heat; and the spontaneous fission neutron source. Calculations have been performed for a typical swimming pool-type research reactor, using materials test reactor-type low-enriched uranium fuel, for four different operating cycles. A fuel burnup range of 5 to 35% has been considered. The cooling time ranged from a fraction of a second to thousands of years. Results indicate that the amount of plutonium produced is strongly dependent on fuel burnup. It is not significantly affected by the operating cycle. The operating history of the reactor has a strong influence on fission product inventory and decay heat. The main contributors to activity and decay heat for the first two to three centuries are fission products; thereafter, actinides are the main contributors. The activity and decay heat drop drastically during the first day after shutdown. Both alpha activity and the spontaneous fission neutron source are strongly dependent on the operating cycle and fuel burnup. These increase with an increase in the fuel burnup but decrease for a longer power-on cycle.