The development of a fuzzy logic-based barrier (FLB) method for the evaluation of the proliferation resistance of nuclear fuel cycle systems is described in this paper. The method is based on using a group of system-dependent, measurable, or quantifiable variables to define the proliferation barrier effectiveness of a system as fuzzy numbers. The usefulness of the FLB method and the resulting metric in quantifying the proliferation resistance of fuel cycle systems was also investigated by applying it to three fuel cycles, i.e., light water reactor-once-through, light water reactor with mixed oxide fuel, and direct use of spent pressurized water reactor fuel in CANDU reactor. To address the issue of subjectivity in assigning barrier weighting factors or fuzzy numbers, the sensitivity of the results to the definition of fuzzy numbers and weighting schemes was also investigated.