The effect of assumption of plane state of stress on the predictability of experimental results observed during the mode II stable crack growth (SCG) through 8 mm thick compact tension specimens (CTS) of a workhardening aluminum alloy (D16AT) has been studied. Experimental results include load-sliding displacement diagram, extent of SCG, crack front geometry and fracture surface fractographs. The experimental results show that the crack extends in its own plane, the fracture surface is flat, smooth and free of any shear lip. The crack front geometry, which is straight initially, remains mostly so throughout the SCG. Theoretical investigations have been done using an elastic-plastic finite element scheme and the COA/COD criterion as the criterion governing the growth. Finite element results, assuming plane stress and plane strain conditions separately, on the load-sliding displacement diagrams, J-resistance curve, plastic zones and variation of equivalent stress and strain along the crack-line ahead of the crack tip are also presented. The resistance curve is a straight-line and the magnitudes of equivalent stress and strain increases as the crack extension proceeds. In general, the predictions based on the assumption of plane state of stress are closer to the experimental results.