This paper describes the concept of developing a component program library for building energy simulation and the current state of its development, which has been made by a working group of theSociety of Heating, Air-Conditioning and Sanitary Engineers, Japan (SHASEJ) since 1985. The purposeof developing this component program library is to assist researchers and engineers in designing theirown programs for their own objectives. Since the early 1970s, quite a few building energy simulation programs have been developed: for example, NBSLD, DOE-2 and BLAST are internationally well known programs and, in Japan, programs called HASP and ACSS are also available. Although these programs are usually suitable for simulating a normal building, it is not often that they can automatically be used to simulate new energy saving strategies in air-conditioning systems, innovative window systems and so on. To accomplish such simulations, one usually must modify portions of the computer program being used. This necessarily causes a certain amount of tedious work and occasionally seems almost impossible. These problems may be resolved by the use of the component program library for building energy simulation. Each component program in the library described in this paper has been written in FORTRAN 77 and designed so that it has the fewest number of lines as possible (the average is about 30 lines. A program having more than 100 lines is not allowed.). Every component program's documentation describes how to use it, an example of its use, fundamental equations involved, its limitations and the whole list. So far, this component program library contains approximately 180 programs. During the course of development, we found that many component programs can be classified in a tree-like structure. This suggests that a component program library may have a potential for constructing a simulation program to conveniently and accurately model a room or building. In other words, this approach may help us make a program which will simulate a room or building as we assemble building elements and various heating/cooling system components to construct the room or building.