Various nominal proton beam energies from 20 to 45 MeV in 5 MeV steps were determined by measuring each range using a homemade range measurement device for accelerated high energy ion beams that were produced by the MC-50 cyclotron at the Korea Institute of Radiological & Medical Sciences (KIRAMS). First, we carried out the range measurements for incident protons with three dierent stopping materials, aluminum, carbon (graphite), and Lexan (polycarbonate), with densities of 2.7, 1.8, and 1.2 g/cm 3 , respectively, which were employed in order to measure dierent ranges for various proton beam energies. Also, we conducted the MCNPX simulation under conditions identical to those of the above experimental environment. Finally, we determined the proton beam energy by applying the measured experimental ranges to the simulated results, and we compared them with well-known data like SRIM 2008 and ICRU Report 49. From the results, the newly-determined proton energies and the nominal ones were found to be in good agreement, within statistical error. In this work, the results for the proton energies determined applying MCNPX simulations show that the developed device for ion beam range measurements is very useful for determining the energy because of its accuracy, rapidity and convenience.