We herein describe a needle insertion method involving tissue preloading for accurate breast tumor treatment. A mechanical preloading probe locates a tumor lesion from ultrasound imaging information and reduces lesion displacement during needle insertion by pressing the breast tissue. We validated the insertion accuracy of this method by numerical simulation and experiments both in vitro and in vivo. For further accuracy enhancement, we evaluated the geometry effect of the preloading probe on needle insertion accuracy by experiments in vitro. We compared the insertion accuracy between insertion with preloading using different probe diameters and normal needle insertion. In addition, we compared insertion accuracy at different tumor depths. The data indicated a tendency for adaptation of larger preloading probe diameters with deeper tumors. This suggests the potential for our method to enhance placement accuracy by real-time geometry regulation.