The depth dependent UV-light absorption profile of GaN nanorods with different lattice arrays and filling factors was studied using finite-difference time-domain (FDTD) methods. By comparing to the results from LambertBeer's law with Maxwell-Garnett effective medium theory, we identified the quantitative contribution from nano-scattering effect on the light absorption in the nanorod arrays. The FDTD study of graphical 3D profile of light absorption and electric field intensity was parallelly conducted to investigate the origin of the nano scattering. We found that the coupled electric field in the gap regions led to the larger absorption crosssection of the nanorod arrays, which is attributed to the distorted depth profile of the light absorption.