Enhancing the proliferation resistance of a nuclear reactor system is of significant interest to advanced reactors or fuel cycles development. There have been numerous approaches suggested to date to quantitatively assess proliferation resistance of a nuclear reactor or fuel cycle, with no consensus on the use of these methods yet. This paper presents a fuzzy logic-based barrier method for quantitative assessment of proliferation resistance of nuclear reactor systems or an entire fuel cycle. The method is based on the barrier framework (BF) developed by the TOPS (Technical Opportunities To Increase the Proliferation Resistance of Global Civilian Nuclear Power Systems) task force of the DOE Nuclear Energy Research Advisory Committee. The qualitative TOPS BF approach was rendered to a quantitative analysis method by using barrier effectiveness functions for each respective barrier under consideration. The imprecise and uncertain nature of the assessment was addressed by using a fuzzy logic. To minimize the influence of subjective judgments in data interpretation, measurable or quantifiable metrics were used to the extent possible in characterizing barrier effective functions and in the aggregation of barrier functions/metrics. The method was applied to quantify and compare the proliferation resistance of major nuclear reactor systems against a proliferation threat by a "developing" nation.