The synthesis of a network of crosslinked carbon nanotube/hemoglobin (CNT/Hb) on a thiol-modified Au surface and its use as sensor for H₂O₂ is reported. The constructed CNT/Hb 3-D network exhibits high conductivity by expediting the electrical transfer from Hb to the Au electrode and has a high electrocatalytic property for sensing hydrogen peroxide (H₂O₂). CNTs were first oxidized by treatment with concentrated nitric acid for 10h and functionalized by reaction with the thiol group of 4-aminothiophenol and Hb was also reacted with 4-aminothiophenol. These modified CNTs and modified Hb were immobilized on a 4-aminothiophenol monolayer-modified Au electrode by co-electropolymerization by repetitive cyclic voltammetry scans ranging between -0.1 and +1.1 V (versus Ag/AgCl). Cyclic voltammogram (CV) and amperometry were employed to study electrochemical properties of the modified electrodes. Direct electrical communication between the redox center of Hb and an Au electrode was established through 3-D network of crosslinked CNT/Hb. The Hb present in the 3-D CNT/Hb network exhibited a pair of quasi-reversible redox peaks with a midpoint potential of -0.225 V and -0.075, cathodic and anodic respectively. The electron transfer rate constant, K(S) and electron transfer co-efficient α were found to be 0.51 s⁻¹ and 0.58, respectively. The modified electrode was used as a biosensor and exhibited a high sensitivity, long linear range and lower detection limit to H₂O₂, under optimal conditions. The apparent Michaelis-Menten constant (K(m)) and Hb adsorption in the CNT/Hb network with average surface coverage of were found to be 0.19 mM and 4.8×10⁻¹⁰ mol cm⁻², respectively. This system should be very useful for other sensing applications.