We　report　the　thermal　tuning　of　the　mechanical　nonlinearity　in　GaAs　double-clamped　MEMS　beam　resonators　for　sensitive　thermal　sensing　applications.　We　have　estimated　the　mechanical　nonlinearity　in　GaAs　MEMS　beam　resonator　by　measuring　its　resonance　frequency　as　a　function　of　oscillation　amplitude.　The　MEMS　resonator　shows　a　hardening　nonlinearity　with　a　small　linear　oscillation　range　of　&　SIM;30　nm.　When　electrical　heat　is　applied　to　the　MEMS　beam,　we　have　observed　a　significant　reduction　in　the　mechanical　nonlinearity　of　MEMS　resonators　near　the　buckling　point　of　the　MEMS　beam.　The　decrease　in　the　mechanical　nonlinearity　originates　from　the　bending　of　the　MEMS　beam,　which　gives　a　softening　nonlinearity　term　and,　hence,　compensates　the　total　nonlinearity.　With　the　thermal　tuning　effect,　MEMS　resonator　can　maintain　a　very　large　quasi　linear　oscillation　amplitude　of　&　SIM;300　nm,　which　is　&　SIM;10　times　larger　than　the　linear　oscillation　range　without　the　control　of　nonlinearity.