RUP(Rational Unified Process) is a development process which is based on object-oriented, use case centric, architecture centric, and iterative approach. Public projects performed previously adopt waterfall lifecycle model for development of large scale software. However, recently various projects adopt an iterative approach to minimize risks of a project and to enhance quality of software. But few research result on practices of RUP as the subprocess of system development process is available. This paper presents a system development process which uses RUP as the subprocess for a subset of the system. Thus we introduce a tailored RUP for K-CIS(Korean Core Instrumentation System). Moreover, we assess the application result of K-CIS with typical waterfall lifecycle model and RUP. We believe that the results of our work are useful for tailoring a system development process with RUP, reducing risks of development, and enhancing the quality of a final product.