Most domestic fossil power plants have exceeded 100,000hours of operation with the severe operating condition. Also, cyclic operation of the plant subjects the piping system to mechanical and thermal fatigue mechanisms and poor of defective support assemblies can impose massive loads onto the piping system and induce unstable piping displacement. In order to prevent the serious damage and failure of the piping system in fossil power plants, 3-dimensional displacement measurement system was developed for the on-line monitoring. Displacement measurement system was developed with LVDT and rotary encoder type sensors. This system was installed and operated on a real power plant successfully.