Producing what customers need with near mass production efficiency, or mass customization, has been a major trend in industry. Effective definition of customer requirements is a pre-requisite for realizing mass customization. The new method, called "Design by Customers", is proposed. The proposed method is an approach for companies to communicate to customers about what the company can offer, to find out customer needs, to assist customers in making choices and to negotiate for agreements. In this paper, the design and manufacturing capabilities in making company are represented in a Product Family Architecture. Adaptive Conjoint Analysis is then applied help customers assert their needs, defined variations from base product, visualize their options and assess alternatives. A power supplier of a electronic appliance is shown as an example to demonstrated the suggested method.ed method.