The relative effects of thickness and material strength level on the in-plane forming limits of thin sheets of a low carbon Deep Drawing Steel has been experimentally investigated. To investigate the effects of sheet thickness at constant strength, the sheet material was chemically thinned from a starting thickness of either 0.318 mm or 0.310 mm to various levels down to 0.152 mm. In order to establish the effect of material strength level at a given thickness, the starting material was cold rolled to different thicknesses. Strength effects were deduced by comparing results of tests from cold rolled material with chemically thinned material at different thickness levels. Uniaxial tensile elongations and plane strain limit strains appear to be relatively unaffected by changing thickness at a given strength level, while the balanced biaxial stretching limit strain decreases gradually with decreasing thickness at constant strength. Increasing strength, however, dramatically reduces the tensile elongation and plane strain limit strain, while the balanced biaxial stretching limit decreases more gradually with increasing strength, but more rapidly than the balanced biaxial stretching limit strain decreases with decreasing sheet thickness at constant strength. As far as the authors are aware this is the first paper to investigate the effect of sheet thickness on formability when the strength of the sheet has been held constant as the thickness of the sheet is varied.