Grid computing is becoming a mainstream technology for large-scale distributed resource sharing and system integration. In grid environment, everything can be regarded as a grid service. Workflow technology has grown to be an important method to manage processes and resource. Building a distributed workflow management system (WFMS) based on grid services brings many attractive characteristics such as great efficiency, higher fault tolerant ability, good load balancing and reliability etc. This paper presents the architecture of s-WFMS, a service-based workflow management system in grid environment. In s-WFMS, most functional components of a WFMS are abstracted and evolved to high-level grid services. A feasible implementation method of the architecture is also presented.