A new interpretation of the relationship between the saturation exponent (n) and the water saturation (S w ) of a reservoir rock is presented. Based on the capillary theory and Purcell equation, the relationship between resistivity index (I) and the permeability (k), water-phase permeability (k w ) has been obtained. By combining SDR model, such relationship can be transferred into a novel model of probing the saturation exponent under variable saturation conditions. Based on the theoretical derivation, NMR experimental measurements of core examples were implemented to testify the reliability brought by this development. Laboratory measurements of transverse relaxation were carried out on core samples from a formation in the Middle East. NMR experimental measurements were made with 50 samples fully water saturated. Measurements at various air/water saturation (i.e., at residual water saturation) were made on 16 samples among them obtained by different centrifuging speeds. The experimental results show that the calculated saturation exponent values and water saturations follow a power law relationship which is in accordance with researches have been published about that n is not a constant but a variable parameter dependent on water saturation, quantitatively. This study has inferred the quantitative relationship between n and S w through NMR information firstly.