A study to determine the optimum pin diameter for carbide fuel in a liquid-metal fast breeder reactor was made. Several subsidiary parameters, including coolant velocity, burnup, and properties of the clad and subassembly duct material, were studied for their effect on the optimum pin diameter. The materials used in the analysis were Type 316 stainless steel and a high strength, low swelling material capable of operation to a damage fluence of 3.6 × 1023 n/cm2. Using doubling time and fuel cycle cost as the basis for judgment, an optimum pin outer diameter (o.d.) range of 0.37 to 0.40 in. was found for both the near-term and advanced structural materials. Because of practical limits of the Fast Test Reactor testing, an o.d. of 0.37 in. is recommended. The optimum pin diameter was found to be independent of fuel burnup. From a doubling time point of view, no incentive was found for development of carbide fuel with a burnup greater than 80 to 90 MWd/kg.