In this article, we report a geometric study of the contact area for the hardness test of gold bonding wires using nanoindentation testing equipment based on the Oliver and Pharr method (OP method), which is frequently used to measure the hardness and elastic modulus. Since the OP method assumes that the sample surface is flat, the contact area detected by the OP method is underestimated, especially for a curved surface. We developed a contact area correcting method for nanoindentation testing, considering the geometric configuration during tip indentation on a curved surface, by theoretical analysis and experiment using nanoindentation testing equipment. Analysis results show that the area ratio (real contact area/contact area detected by the OP method) significantly decreases with decreasing wire diameter and increasing displacement into the surface, and we found the offset values to correct the contact area detected with the OP method for different wire diameters. Furthermore, nanoindentation testing was carried out on commercially available gold bonding wires with diameters of 30 and 50 µm. Measurement results show that the slopes of hardness–displacement curves slightly decrease with increasing displacement into the surface owing to the underestimated contact area. Then, we applied the analyzed offset value to the measured contact area to correct the obtained hardness. The corrected hardnesses of 30- and 50-µm-diameter wires are approximately 0.8 and 0.9 GPa, respectively. These corrected values show an increase of 25–30% compared with the uncorrected values.