With the development of grid and grid applications, more and more flow jobs have appeared. Definition, execution and management of jobs need the support of software system and the job manager is the core component. The job manager interprets the job definition and provides the monitor function. A job description language is proposed and a job manager is designed based on early work of Web service composition. The job manager is composed of invoker, conversation manager, instance manager, job instance, activity instance and activity invoker. The job manager provides an executing engine for grid jobs. The layered architecture of the job manager simplifies the message processing. Three different conversation mechanisms supported by the job manager satisfy the requirement of the conversation between jobs and between jobs and component services. Event-based message exchange mechanism makes the relation among components of the job manager clearer and meets the requirement of the object-oriented design.