This paper argues about the partitioning in hardware/software co-design and suggests the methodology applying extreme programming to complement the co-design. This approach, contrary to complex method in the existing development environment of embedded systems, arises from need of agile methodology in latest development environment which requires time to market. Through adding a guide line of extreme programming to the advantage of co-design, the synergy effect of general process is expected. The risk management such as the cost management and the time management is suggested on the basis of PAMUX(Partitioning Methodology Using eXtreme Programming) and also the action plan such as the group organization and process establishment is suggested on the basis of the methodology which increases the reliability of partitioning. Finally, traits and case study of suggested methodology are evaluated by Lego robot project and it allows the PAMUX to add reliability.