Minimally invasive pedicle screw placement (MIPSP) is a widely used treatment for spine diseases. When coupled with intraoperative navigation modalities, robots may help improve surgical outcomes and reduce complications. With such a system, the application of pedicle screws has been expanded from needle insertion to the spine surgery. This paper investigates the possibility and feasibility of robot-assisted MIPSP based on ultrasound (US) guidance. The proposed system is non-radiative and fiducial-free, using purely image information to close the registration loop. Then the system automatically positions the drill tip to a planned screw trajectory and executes the operation. Experiments were conducted on both ex-vivo lamb and human cadaver spines. An entry point accuracy of 2.39±1.41 mm, and orientation accuracy of 2.82±1.85∘ was found for 24 drilled trajectories on three lamb spines. On the ex-vivo human spine, the position error averaged 3.08±2.43 mm at the entry point and 4.05±2.62 mm at the stop point across 16 drilling instances. Moreover, a 87.5% success rate was reported by using Gertzbein-Robbins grade. The experimental results demonstrate the potential for offering a radiation-free alternative. Although restricted to cadaver trials, this work encourages further exploration of this technology to assist surgeons in maximizing performance in clinical practice.