This paper presents a novel visual tracking control scheme, which is robust to the quantization error in practical implementation. The proposed control scheme is based on an error model of camera-object visual interaction in image plane for unicycle mobile robots. In order to overcome the quantization error encountered in practical system, a stability necessary condition for ensuring global asymptotic stability of the closed-loop visual tracking system is derived through Lyapunov's direct method. The robust control law is then proposed to guarantee that the visual tracking system satisfies the stability necessary condition based on Lyapunov theory. Experimental results verify the effectiveness of the proposed control scheme, both in terms of tracking performance and system convergence.