Many Grid work ow middleware services require knowledge about the performance behavior of Grid applications/services in order to eectively select, compose, and execute work ows in dynamic and complex Grid systems. To provide performance information for building such knowledge, Grid work ow performance tools have to select, measure, and analyze various performance metrics of work ows. However, there is a lack of a comprehensive study of performance metrics which can be used to evaluate the performance of a work ow executed in the Grid. Moreover, given the complexity of both Grid systems and work ows, semantics of essential performance-related concepts and relationships, and associated performance data in Grid work ows should be well described. In this paper, we analyze performance metrics that performance monitoring and analysis tools should provide during the evaluation of the performance of Grid work ows. Performance metrics are associated with multiple levels of abstraction. We introduce an ontology for describing performance data of Grid work ows and illustrate how the ontology can be utilized for monitoring and analyzing the performance of Grid work ows.