Liaison is a workflow management model that supports complex workflows and human resources management. Liaison System is the implementation of the Liaison Model. This paper discusses the architecture of the Liaison Workflow Engine, the core of the Liaison Workflow Management System, which is responsible for controlling workflows. Besides managing and controlling complex workflows and human resources, this Liaison Workflow Engine architecture is designed to have high flexibility so as to cope with heterogeneous distributed systems and different kinds of distributed workflow applications. The Liaison Workflow Engine consists of five major components, namely: Scheduler; Task Manager, Actor Manager, Information Manager and Actor Interface Manager. These components are responsible for workflow management, task management, job assignments, information management and interface management respectively. There are also Exception Handlers associated with Scheduler, Task Manager, Actor Manager and Information Manager to manage and handle exceptions in each of these components.