Abstract: In order to compare the advantages and disadvantages between natural uranium and slightly enriched uranium, and determine the most optimized slightly enriched uranium level, the fuel cycle cost, natural uranium consuming and high-level waste accumulation are studied for CANDU-6 reactor with various enriched level fuel. The codes used in this study are WIMS-AECL and ORIGEN2. The results show that the slightly enriched uranium can save natural uranium of more than twenty percent, reduce spent fuel accumulation of more than thirty percent and cut down fuel cycle cost of fifty percent. The study indicats that 1.3% is the most optimized enriched level.