Knowledge of thermal conductivity of fiber reinforced composites is important to understand the thermal gradients and heat build-up during cure. Thermal conductivity of composite materials depends on temperature, direction of heat flow, orientation of fibers, stacking sequence of the laminate and state of cure. The purpose of this study was to study thermal conductivity of carbon/epoxy composites as a function of temperature, state of cure, and direction of heat flow and finally to determine semiempirical parameters of the Halpin-Tsai model. Determination of Halpin-Tsai's reinforcement efficiency parameter, ‘ζ’ is essential to predict thermal conductivities of fiber-reinforced composites and its constituent materials without actually testing specimens. The test specimens were manufactured from Cycom 5320-8HS prepreg and 5320-1 epoxy resin. 5320-8HS is toughened out-of-autoclave (OOA) prepreg which is reinforced by T650 3K carbon fiber in 5320-1 epoxy resin. The test specimens manufactured from these materials had different degrees of cure (DOC). The thermal conductivity, specific heat and diffusivity tests were performed by Laser Flash technique. Differential Scanning Calorimeter was employed to verify the specific heat and to determine DOC of the samples. Results show that, ‘ζ’, increased by about 36 % when DOC increases from 39% to 95% along 0° direction. Along the TT direction, ‘ζ’, increased by about 200% when DOC increases from 39% to 95%. The thermal conductivity and thermal diffusivity in 0° direction increased by 8.7% and 9.5 % respectively when DOC increases from 39% to 95%. The thermal conductivity and diffusivity in the TT direction increased by 11.1 % and 15.0 % respectively when DOC increases from 39 % to 95 %.