Abstract: On the basis of the fundamental theory of statistical mechanics and Eyrings absolute rate theory, a general theoretical model for the description of liquid diffusivity was developed. In this model, the influences of temperature, pressure and composition on liquid diffusivity were comprehensively considered. From the model:① an equation which can be used to correlate liquid self - diffusivity under high pressure was obtained,② a new equation was derived to calculate the mutual diffusivity at different concentrations in binary solutions, ③a general equation was developed for calculating the diffusivity at infinite dilution. All these equations were applied to practical liquid systems and the calculating results showed that the equations proposed has a higher accuracy in calculating liquid diffusivity than those of literature.