Abstract: Workflow technology plays an important role in the development of critical business applications. According to the analysis of the requirements to develop critical business applications, a framework of a relationship based lightweight workflow engine is presented, which is built over the conventional relational database system. It consists of three components, namely: organization model, information model and control model. The reasons that the concepts of relationship and lightweight are adopted to design the workflow engine are thoroughly discussed. The principle to design, the representation of and the way to implement the organization, information and control model of the workflow engine are described respectively. A prototype of this workflow engine has been applied to a project about the automation of trademark registration and administration, which shows the application of the workflow engine can markedly shorten the cycle of the development critical business applications.