To evaluate the performance of nuclear fuel, it is necessary to measure the fission gas release, fuel temperature, fuel stack elongation, cladding elongation, fuel rod inner pressure, coolant temperature, and neutron flux during irradiation tests. Linear variable differential transformer (LVDT) is applied to the in-pile instruments for measuring the fuel rod inner pressure. In this study, design modification of LVDT was carried out to measure the fuel elongation and cladding elongation, without changing the structural soundness of the conventional nuclear fuel test rig used in HANARO. The magnetic core was directly connected to the fuel stack or cladding using a supporter, and it moved through the guide rail of LVDT sensor according to the deformation of fuel pellets or cladding. The performance verification and data calibration of the modified LVDT were verified by the deformation simulation system, where displacement was induced using a micrometer.