We describe a palpation-based needle insertion method for diagnostic biopsy and radiofrequency ablation treatment of a breast tumor. The mechanical palpation probe locates cancerous tissue from ultrasound imaging information and reduces tissue displacement during needle insertion by pressing the breast tissue. We examined the insertion accuracy of this method and compared palpation-based needle insertion with normal needle insertion both in vitro and in vivo. Palpation-based needle insertion had a smaller error, suggesting that this procedure is a safe, effective alternative to traditional methods of breast needle insertion.