The paper presents comparison of two methods of voxel defining for trabecular bone structure modelling. Regular cubic voxels were considered, the size of which changed uniformly in three mutually perpendicular directions depending on the adopted resolution. Also elongated rectangular voxels were proposed, characterized with fixed in-plane resolution and variable length along out-of plane direction. For both types of voxels a number of analyses were performed using finite element method with structures of varied BV/TV values. It was stated that voxel dimension change in out-of plane direction allows for decreasing of a number of scans required for correct reflect of the modelled structure stiffness for the needs of numerical analyses.