Abstract: A fractional sub-loading surface model for clays is developed in the present study. The fractional plastic flow rule adopted in the proposed model is able to account for the non-normality of the flow direction with respect to the yield locus without introducing a plastic potential. Hence, a unified description of the associated and non-associated plastic flow rules is achieved. A stress-induced fractional dilatancy rule can be conveniently derived through the fractional plastic flow rule to consider the effect of the over-consolidation ratio on the dilatancy of clays. The analysis shows that increasing the over-consolidation ratio will reduce the dilatancy under a constant loading pressure. Compared with the modified Cam-clay model, the proposed model introduces only one extra dilatancy-related parameter and can describe the strain-softening and dilatancy features of over-consolidated clays. Model predictions show good agreement with the experimental results, indicating the capability of the proposed model in describing the behavior of clays.