In　order　to　reveal　the　dynamic　behaviors　of　a　planetary　gear　train　under　a　real　working　environment,　a　pure　torsional　nonlinear　dynamic　model　was　established　by　considering　gear　backlashes,　time-varying　meshing　stiffness,　transmission　errors,　and　sliding　frictions　between　gear　pairs.　The　differential　equations　of　motion　of　the　transmission　system　were　derived　by　Newton’s　second　law　and　then　solved　by　Runge–Kutta　method.　The　nonlinear　dynamic　characteristics　of　the　planetary　gear　train　were　classified　into　quasi-periodic　response　and　chaos　response　based　on　the　numerical　simulations　of　time　history　plots,　phase　plane　plots,　Poincare　maps,　and　Fourier　spectra　diagrams.　The　results　indicate　that　the　sliding　friction　brings　complicated　nonlinear　effects　into　the　planetary　gear　train　dynamics.　With　the　increment　of　the　friction　coefficient,　friction　forces　between　gear　pairs　cause　more　power　loss,　which　makes　the　sys-tem’s　response　become　more　regular.　As　a　result,　the　planetary　gear　system　experiences　a　chaos　response　and　a　quasi-periodic　response　sequentially.　©　2017　Taylor　&　Francis　Group,　London.