Demand for high-quality silicon wafers poses a tremendous challenge to wafer manufacturers in today's competitive environment. Among various quality-improvement activities, process capability index C_pmk is widely used in industry to measure the ability of firms or their suppliers to meet quality requirements, because measurements of product quality often cannot be precisely recorded or collected. This study uses fuzzy numbers to construct a fuzzy estimator for C_pmk. Moreover, a testing procedure and a procedure for ranking multiple processes are provided to solve the wafer-supplier-selection problem.