We study the correlation heights, which indicate the formation height of Extreme Ultraviolet (EUV) lines in an active region using observations from the EUV Imaging Spectrometer and Solar Optical Telescope on board Hinode. The nonlinear force-free field optimization method is adopted to extrapolate the three-dimensional magnetic fields to higher layers. Three subregions with different characteristics are selected in the active region for this study. The results show that the formation heights in different subregions vary with their different magnetic fields or velocity patterns. After solving the line blending problem between the He II 256.32 Å and Si X 256.37 Å lines by the double Gaussian curve fitting, we find that the transition region lies higher in a strong magnetic area. In a preflare heating area, there possibly exist multithermal loops as implied by comparing the Doppler velocity and the magnetic field on the solar disk.