Nowadays, outsourcing as a strategic goal is becoming a critical success factor used by different organizations in order to provide high quality, and low price products for their customers. In this context, supplier selection has become an extremely important decision for companies. Management and human resource researchers have focused more on qualitative aspects and conceptual models, whereas engineering science researchers have developed quantitative and full-mathematical models and techniques. Since it is important to consider both aspects of supplier performance, this paper develops a fuzzy-based model to evaluate the leanness of supplier performance. The model presented here, gives a single unit-less score, which clearly shows the leanness of supplier performance. Based on the final score, managers can easily decide on supplier selection. The model includes a systematic measurement of supplier performance with six characteristics: flexible, dynamic, fuzzy logical, continuous improvement, easy to use, and single unit-less (dimensionless) score.