This paper shows the procedure to decide an optimal design principle for improving the disassemblability with considering of disassembly conditions. On the bssis of the disassembly mechanism of products and the structure of parts and subassembly, the disassemblability is classified into four categories: graspability, accessibility, transmission of disassembly power and handling. The weighting values of the influential factors are calculated by the method of AHP(Analytic Hierarchy Process). The disassemblability is evaluated quantitatively. We established some score tables for the evaluation. Using these score tables, several principles for higher disassemblability in accordance with work conditions can be decided. An optimal design principle can be found by the comparison with the total scores of some disassembly conditions.