The process of program debugging is essentially an intelligence intensive process. It is thought viable to develop a knowledge-based tool to help programmer perform this process. This paper presents the design of such a system. Unlike other knowledge-based debugging tools which are mostly based on formal program specification and automatic program understanding, this tool is based on debugging heuristic. This tool is a debugging assistant which only suggests the programmer in program debugging what and where to examine using the debugging heuristic stored in the knowledge base. In this paper, a umber of useful heuristic debugging knowledge are explained and their usage in debugging process are described. The, a scheme to organize the knowledge in the knowledge base and an intelligent program debugging assistant using the knowledge are proposed and discussed.