Insulating　polymers　have　been　recognized　as　a　promising　class　of　gate　dielectrics　for　organic　field-effect　transistors　(OFETs).　However,　the　relative　permittivity　of　most　single　polymer　dielectrics　is　quite　fixed　and　too　low　to　afford　low-operating　voltages　of　OFETs.　For　flexible　low-voltage　OFETs,　dielectric　tunable　polymer　composites　are　highly　desirable.　Here,　a　new　type　of　binary　polymer　composite　dielectrics　is　developed　by　incorporating　a　small　amount　of　polyacrylic　acid　(PAA)　into　poly(methyl　methacrylate)　(PMMA)　to　reduce　the　operating　voltage　and　enhance　the　device　performance.　The　resulting　dielectric　layers　deliver　a　tunable　relative　permittivity　from　3.32　to　4.28　with　an　increase　of　PAA　contents　in　the　composite.　As　results,　flexible　OFETs　using　PMMA:　PAA　dielectrics　show　greatly　improved　mobility　and　reduced　threshold　voltages　with　a　low-operating　voltage　below　-5　V.　The　OFETs　using　the　composite　dielectric　also　exhibit　excellent　performance　stability　during　mechanical　bending　tests　with　different　radii,　where　the　mobility　can　maintain　95%　of　its　initial　value　over　5000　cycles　under　a　bending　radius　of　5　mm.　The　tunable　dielectric　properties　and　high　robustness　of　these　novel　dielectrics　make　them　promising　candidates　for　solution-processed　low-voltage　flexible　OFETs　with　low　power　consumption.