Ultrasonic-assisted drilling (UAD) is employed to solve the problems of large drilling stress, difficulty of chip breaking and removal, and poor machining quality in micro drilling of stainless steel. Drill trajectory and axial cutting thickness in conventional drilling (CD) and UAD processes were analyzed. The effect of ultrasonic vibration on drilling force, chip curling radius and hole morphology were investigated based on the finite element simulation and experiment results. The results indicate that compared with CD, the effective cutting time and cutting stress under chisel edge are reduced significantly, the chip curling radius is smaller and thus chip is easier to break, the thrust force and entrance burr are reduced, and micro-hole quality is improved in the UAD process.