Abstract: At present many viewpoints on problem difficulty are partial. Based on these viewpoints a Relational-Representational Complexity Model (RRCM) was developed. In this model, two concepts of "problem difficulty" and "problem solving difficulty" were distinguished. The essence are relational complexity and representational complexity respectively. Relational complexity includes horizontal and hierarchical complexity; Representational complexity includes width and depth of representation. Test of internal and external validation of this model based on the data from 172 4~6th graders’ performance on constructing problems and judging the conditions of calculating area-of-rectangle and solving this kind of problems suggested which could interpret the order of problem (-solving) difficulty and discriminate the levels of representational complexity of excellent, normal and poor students. Indicating that this model has theoretical value

Key words: relational-representational complexity model, problem (-solving) difficulty, knowledge base, internal or external validation