Contact　stiffness　and　backlash　in　the　harmonic　drive　significantly　impact　a　robot＇s　positioning　accuracy　and　vibration　characteristics.　The　height　of　the　harmonic　drive　tooth　pair　is　typically　less　than　1 mm,　making　the　measurement　and　modeling　of　backlash　and　contact　stiffness　inherently　complex.　This　paper　proposes　a　contact　stiffness　and　backlash　model　by　establishing　a　correlation　between　fractal　parameters　and　tooth　contact　load.　To　obtain　the　fractal　roughness　parameters　of　the　real　machined　tooth　surface,　a　combination　of　a　noncontact　optical　profiler　and　the　RMS　method　is　employed.　Subsequently,　the　study　explores　the　influence　of　rough　tooth　surface　and　contact　force　fractal　parameters　on　contact　stiffness　and　gear　backlash.　The　results　demonstrate　the　substantial　impact　of　surface　topography　parameters　and　contact　force　on　contact　stiffness　and　backlash.　Specifically,　an　increase　in　the　fractal　dimension　correlates　with　a　reduction　in　gear　backlash　and　contact　stiffness.　Conversely,　the　fractal　roughness　parameter　exhibits　the　opposite　effect.　Notably,　an　increase　in　contact　force　enhances　contact　stiffness.　©　IMechE　2024.