This paper proposes a development process of expert systems by using a library of re-usable methods which supports the processes both in specifications and in implementations for the development of expert systems. The objectives of this paper are to integrate the processes of specification and implementation into a single process, and to cut off the cost of the development of expertsystems. The methods of the library are organized into an executable specification language REPOSIT (REusable Pieces Of Specification-Implementation Templates) which combines two types of expressions: one is the relationship among input, output and reference knowledge, and the other is a prolog-based representation. A specification description of expertsystems is given as a knowledge-flow graph which is comprised of REPOSIT methods and knowledge names. Each method consisting an expertsystem's specification can be refined into a combination of fine-grained methods. In order to support the step by step operationalization of abstract models into detailed implementation descriptions, the following methods are given: a) classifying the adding patterns of control structures, b) standardizing the knowledge management and c) providing refinement policies. Finally, through the application of our proposed method to expertsystems' development, we have confirmed that REPOSIT improves the re-usability of implementation codes in the maintenance of expert systems