Abstract: CLTS (C Language Tutoring System) is an intelligent computer assisted instruction systemfor C language programming.In this paper,the architecture,the function description and implementa-tion of the CLTS are presented A hybrid representation scheme of combining semantic network andproduction-rule is used for the domain knowledge.The instruction decisions correspondingly use two-step reasoning strategy based on semantic network and production-rule.The student model has the func-tions of diagnosing error and dynamically renewing.The natural language interface enhances the inter-active flexibility.