Problem-based learning(PBL)is an important part of creative education in medical colleges.Choice and design of cases are of the vital importance to success or failure of PBL course.To enhance students' ability of independent,creative thinking,and their ability of analyzing and solving problems,the roles of primitive cases and model cases as well as interrelation between them were discussed respective- ly.Moreover,five basic principles to be followed in model case design for PBL in Medical Microbiology and Human Parasitology,i.e. objec-tivity, flexibility,consistency,illumination and relevance,were proposed in this study.