A label-free immunosensor based on antibody-modified graphene field effect transistor (GFET) was presented. Antibodies targeting carcinoembryonic antigen (Anti-CEA) were immobilized to the graphene surface via non-covalent modification. The bifunctional molecule, 1-pyrenebutanoic acid succinimidyl ester, which is composed of a pyrene and a reactive succinimide ester group, interacts with graphene non-covalently via π-stacking. The succinimide ester group reacts with the amine group to initiate antibody surface immobilization, which was confirmed by X-ray Photoelectron Spectroscopy, Atomic Force Microscopy and Electrochemical Impedance Spectroscopy. The resulting anti-CEA modified GFET sufficiently monitored the reaction between CEA protein and anti-CEA in real-time with high specificity, which revealed selective electrical detection of CEA with a limit of detection (LOD) of less than 100 pg/ml. The dissociation constant between CEA protein and anti-CEA was estimated to be 6.35×10−11 M, indicating the high affinity and sensitivity of anti-CEA-GFET. Taken together, the graphene biosensors provide an effective tool for clinical application and point-of-care medical diagnostics.