The backlash, which is caused by a gap between an actuator-side gear tooth and a link-side gear tooth, is a common phenomenon in manipulator systems with gear in their joints. However, if the amount of backlash is greater than the maximum allowed amount which is to ensure satisfactory meshing of gear, the system instability may appear in dynamic situations and cause position errors in the gear chains. Hence, in this paper, a nonlinear adaptive controller is devised to cope with the effects due to backlash as well as parameter variation. A so-called semi-global tracking is achieved, and simulation studies are also provided to demonstrate the validity of the proposed strategy.