Abstract: The mismatch of cross-sectional area especially for hard materials will result in distinct two-dimensional effect in SHPB experiment. With finite element method and dimensional analysis, the influence is defined as concave two-dimensional effect which leads to monstrous error in small strain measurement, not as conventional planar two-dimensional effect which can be ignored in this case. The conclusion has some significance for guiding SHPB experiment with relatively small cross-sectional area specimen.