Tempo rating which aims to obtain the accurate tempo for the workers is an important process for time study applications. It is possible to reach the normal time after finding the precise tempo values. Westinghouse method, based on the factors of skill, effort, environmental conditions and consistency, can be considered as one of the well-known methods among tempo rating approaches. However, crisp numerical values with linguistic terms are used for evaluating each factor in Westinghouse method. In this study, a fuzzy rule based methodology is proposed due to the vague environment of decision making process of tempo rating. The developed methodology includes fuzzy rules for determining the evaluation scores of each factor. The proposed rating system is applied in a bus-bar production system so as to demonstrate its validity. The proposed rating system yields high level of conciseness on the basis of predetermined motion time system.