Although satellites are getting smaller and the development duration is getting shorter, it still takes a long time to conduct a satellite conceptual design. The System Engineering Design Tool (SEDT) has been developed to effectively and efficiently design small satellites (ranging from 10kg-class nanosatellites to 200kg-class microsatellite) in order to minimize the amount of labor involved in the conceptual design phase. The present SEDT consists of five design blocks and has some characteristics different from system engineering tools previously developed. First of all, it adopts a top-down design methodology which induces a distributed design architecture. SEDT has also implemented the subsystem design process connected in series. It enables the design order of a satellite system to be formed, based on design parameters given by satellite database constructed from over 200 small satellites launched between 1990 and 2004. Utilization of this database can improve the data reliability by acting as a design reference. SEDT incorporates system budgets, mass, and power as verification parameters, along with characteristic trend equations (CTEs). SEDT can do the conceptual system/subsystem design, analyze the design output, and predict the rough order of magnitude (ROM) development cost in accordance with user's requirements. Especially, SEDT implements graphic user interface (GUI) to provide convenience to the users.