On the basis of the free volume theory and activation energy concept,a fundamental equation whichtakes into account the effects of temperature and pressure has been developed.By introducing differentexpressions for the free volume and activation energy,several equations for fluid diffusion coefficients were derivedaccordingly.With the van der Waals free volume and intermal energy formula,a three-parameter model for fluiddiffusion coeffficients at moderate pressure was obtained.The grand average absolute deviation percent of 345data points (44 systems)for self-and infinite dilute inter-diffusivities is 2.32,against the results of the model ofCohen and Turnbull,4.13.In particular,by means of the modified Carnahan-Starling free volume equation,afour-parameter model with average abosolute deviation percent 2.64(30 systems,644 data points)for theestimation of dense fluid inter-and self-diffusivities at high pressures and in supercritical conditions was derived.The derived model is superior to the method of L On the basis of the free volume theory and activation energy concept, a fundamental equation whichtakes into account the effects of temperature and pressure has been developed. By introducing differential equations for the free volume and activation energy, several equations for fluid diffusion coefficients were derivedaccordingly. the van der Waals free volume and intermal energy formula, a three-parameter model for fluid diffusion coeffficients at moderate pressure was. The grand average absolute deviation percent of 345data points (44 systems) for self-and infinite dilute inter-diffusivities is 2.32, against the results of the model of Cohen and Turnbull, 4.13. In particular, by means of the modified Carnahan-Starling free volume equation, afour-parameter model with average abosolute deviation percent 2.64 (30 systems, 644 data points) for the optimization of dense fluid inter-and self-diffusivities at high pressures and in supercritical conditions was derived. derived model is superior to the method of L