The temperature-induced complex refractive index (CRI) effect of graphene is demonstrated theoretically and experimentally based on a graphene coated in-fiber MZI (Mach-Zehnder interferometer).The relationships between real and imaginary parts of the graphene CRI and temperature are obtained through investigating the dip wavelength and intensity of the MZI interference spectrum changing with temperature,respectively.The temperature effect of CRI of the graphene is also analyzed theoretically.Both experimental and theoretical studies show that the real part and imaginary part of the CRI nonlinearly decrease and increase with temperature increasing,respectively.This graphene-coated in-fiber MZI structure also possesses the advantages of easy fabrication,miniaturization,low cost and robustness.It has potential applications in nanomaterial-based optic devices for communication and sensing.