Understanding　the　nonlinear　visco-elastic-plastic　behavior　and　properties　of　PMMA　thin　films　during　nanoindentation　is　helpful　for　predicting　the　plastic　deformation　of　thermoplastic　polymer　products　during　manufacture　and　evaluating　stress　to　predict　the　reliability　and　failure　behavior　of　their　service　structures.　This　article　reports　a　study　on　the　nanoindentation　of　PMMA　thin　films　via　instrumented　indentation　testing　and　numerical　simulation　to　determine　its　constitutive　behavior.　It　is　found　that　both　the　loading　curve　and　the　unloading　curve　at　any　depth　can　be　generated　from　one　indentation　depth　for　the　indentation　tests　of　PMMA　thin　films　with　Berkovich　indenter.　By　coupling　experimental　investigation　and　finite　element　simulation,　we　investigated　the　possibility　of　applying　the　Oyen-Cook　constitutive　equation　to　nonlinear　viscous-elastic-plastic　PMMA　thin　films.　The　results　of　finite　element　analysis　show　that　the　time-dependent　elastic-plastic　deformation　of　the　unloading　curves　becomes　more　conspicuous　when　the　quadratic　viscosity　term　of　the　Oyen-Cook　constitutive　equation　is　decreased.　Finite　element　simulations　using　the　Oyen-Cook　constitutive　model　reproduce　the　experimentally-measured　indentation　load-displacement　curves　with　reasonable　accuracy.　©　2020　The　Authors