To develop an appropriate research platform, this paper presents the design method for a humanoid which has a network-based modular structure and a standard PC architecture. Based on the proposed method, we developed DARwIn-OP which meets the requirements for an open humanoid platform. DARwIn-OP has an expandable system structure, high performance, simple maintenance, familiar development environment and affordable prices. Not only hardware but also software aspects of open humanoid platform are discussed in this paper.