We　have　investigated　the　1:1　internal　mode　coupling　strength　in　a　GaAs　doubly-clamped　MEMS　beam　resonator　by　thermally　tuning　the　frequencies　of　two　neighbored　resonant　modes.　The　anti-crossing　of　the　resonant　frequencies　indicates　that　the　two　modes　couple　with　each　other,　and　the　mode　coupling　strength　was　estimated　by　the　splitting　of　the　anti-crossed　frequencies　of　the　two　modes.　When　the　oscillation　amplitudes　were　tuned　from　the　weakly-driven　linear　regime　to　the　strongly-driven　nonlinear　regime,　we　observed　slightly　increased　frequency　splitting,　indicating　that　the　nonlinear　oscillation　only　makes　a　very　small　contribution　to　the　mode　coupling　strength.　©　2020　The　Japan　Society　of　Applied　Physics.