A Simple Analytical Expression is Proposed in This Article to Calculate the Vibrational Contribution to the Thermal Conductivity of a Polyatomic Fluid. the Analytic Expression Was Obtained based on the Assumption that the Self-Diffusion Process is the Major Mechanism in the Transport of Vibrational Energy. the Proposed Expression is Validated by Comparing the Thermal Conductivity of CO2 Calculated by Molecular Dynamics (MD) Simulations to Experimental Data over a Wide Range of Temperature and Pressure. It is Also Demonstrated that the Proposed Analytic Expression Greatly Increases the Accuracy of Calculated Thermal Conductivity for CO2 at the Supercritical State. © 2010 Taylor & Francis.