Abstract: Oil well cement composites are apt to embrittlement crack downhole, and that will lead to the failure of interlaminar packers of oil well, then effect the exploitation of oil well. In order to solve this problem, it is necessary to reduce the brittleness and increase the toughness of oil well cement composites. Firstly, the dispersancy effectiveness of methylcellulose and carboxymethylcellulose toward carbon fibers were investigated. Then, the effects of carbon fibers on the compressive strength, flexible strength and splitting tensile strength of oil well cement composites were investigated, and the uniaxial and triaxial stress-strain curves of cement composites were measured under the simulated downhole environment. Finally, the microtopographies of carbon fiber reinforced oil well cement composites were observed by scanning electron microscope and the toughening mechanism of carbon fibers toward cement composites was discussed. The results demonstrate that 0.2wt% carboxymethylcellulose solution can disperse carbon fibers effectively. After cured for 28 d, the compressive strength, flexible strength and splitting tensile strength of 0.3wt% carbon fiber reinforced cement composites increase by 8.6%, 31.5% and 52.4%, respectively, under the condition of triaxial direct loading, the elastic modulus reduces by 49.5% comparing with the pure cement composites. The dispersed carbon fibers distribute in cement composites disorderly, form the three-dimensional network structures and toughing cement composites by the energy-dissipation effects of bridging, peel and pulling-out. The conclusions provide theoretical reference to solve the problem of brittleness of oil well cement composites.