Although many workflow models have been proposed, analyses on artifacts are seldom discussed. A workflow application with well structured and adequate resources may still fail or yield unexpected results in execution due to inaccurate artifact manipulation, for example, inconsistency between data flow and control flow, or contradictions between artifact operations. Thus, artifact analysis is very important since activities cannot be executed properly without accurate information. This paper presents a three-layer workflow model for designing a workflow and characterises the behaviour of an artifact by its state transition diagram. By abstracting common usages of artifacts, six types of inaccurate artifact usage affecting workflow execution are identified and a set of algorithms to detect these inaccurate usages in workflow specifications is presented. An example is demonstrated and then related works are compared.