For the increasing complexity of software systems, the main work of software development, maintenance and evolution has been focused on the comprehension of the existing systems. In order to help users comprehend at all aspects and levels of a target system, it is necessary to reversely recover and abstract its high-level architecture, which can reflect the framework and holistic behavioral features of the software system. This paper deals with the problems of architecture recovery from the perspective of process. An approach of extracting Process Structure Graph (PSG) from a target system is presented based on the features of the relations among processes on UNIX platform. First, the static code fragment of a dynamic process can be recognized, then a mapping algorithm that can identify the correspondence between the dynamic process ID and the static process module is given. On the basis of the algorithm, an incremental construction algorithm of PSG and a slicing algorithm for class structure in a process module are implemented respectively. The experimental results show that the extracted PSG is correct, effective and can reflect the high-level structure of the target system in detail at the process level.