This paper presents an innovative approach for the prototyping of a microfluidic chip. By design modularization, unit mold batch fabrication, and mold assembly, the prototyping of polymeric microfluidic chips can be efficient and cost effective. The development of a microfluidic chip for protein reactions, including structure fabrication and chip testing for the initial and modified versions, is reported. Using the proposed approach, design modifications can be realized easily, without remaking photolithography masks or repeating the costly MEMS process. Initial results from testing two micro- fluidic chips (5cm by 1cm and 4cm by 3cm) prove the feasibility of the proposed method and show as well the potential for dramatic savings in time and cost in microfluidic chip prototyping.