The work function (WF) of graphene is an essential parameter in graphene electronics. We have derived the WF of graphene by the thermionic emission method. Chemical vapor deposition (CVD)-grown single-layered polycrystalline graphene on copper foil is transferred to a cross-stacked carbon nanotube (CNT) film drawn from a super-aligned multiwalled CNT array. By decreasing the pore size of the CNT film, the as-prepared CNT-graphene film (CGF) can be Joule heated to a temperature as high as 1,800 K in vacuum without obvious destruction in the graphene structure. By studying the thermionic emission, we derive the WF of graphene, ranging from 4.7 to 4.8 eV with the average value being 4.74 eV. Because the substrate influence can be minimized by virtue of the porous nature of the CNT film and the influence of adsorbents can be excluded due to the high temperature during the thermionic emission, the measured WF of graphene can be regarded as intrinsic.