In this paper, a gear-pair system involving backlash and time-dependent mesh stiffness is examined analytically in an effort to explain the complex behavior of this system. Firstly, the nonlinear dynamic model of a gear-pair system with a periodically time-varying stiffness function and a symmetric backlash is considered. According to this dynamic model, an approximate harmonic solution is obtained which yields mathematical conditions for the occurrence of non-linear behavior in the system response. The stability of the periodic solutions is investigated using Floquet theory. Then the behavior of periodic bifurcation and chaotic motion in the gear-pair system involving backlash and design load is investigated. In some typical parameter regions, the bifurcation diagrams, the chaotic time history and the Poincar