The Assemblability Evaluation Method (AEM) is an effective tool developed by Hitachi, Ltd. to improve design quality for better assembly producibility. The AEM has been widely used by the Hitachi Group as well as by more than 20 other well-known companies around the world. Using this method, in the early design stage, product design quality is analyzed quantitatively and weaknesses in the design's assembly producibility are highlighted. In addition, the effects of design improvements are confirmed with respect to assembly cost. Through these activities, design improvement is realized. In order to improve the functionality and the accuracy of the method to allow a wide variety of use, the extended AEM has been developed. Based on a constructed product and process model, a new evaluation system has been developed and part-based cost estimation has been realized. In this paper, the concept, theory, evaluation procedure, and accuracy of the extended method are described.