Objective: To calculate DDDc (defined daily dose cost ) of external preparation s by the method of finger-tip unit or body surface area, then to judge whether the external preparation could be brought into low price medicine directory . Method ：Reading literatures of research situation s on finger-tip unit and body surface area which applied into calculating daily dose. Result: Some external preparation s that meet the DDDc criterion of low price medicine aren’t brought into low price medicine directory. Conclusion: To perfect the measuring method of DDDc for low price medicine, body surface area proportion should be refined, clinical medicine information ought to be acquired, and rational using dose clinical trial s of external preparation s should be implemented.