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Cu/Ni multilayers with various modulation wavelengths were deposited on a low carbon steel substrates by electrode positing method. Hardness measurements identified that the relationship between the yield strength (one-third of hardness) which increased with decrease of layer thickness for Cu/Ni multilayers and single layer thickness at sub-micron length scale--- could be described by the Hall-Petch (H-P) formula for dislocation pile-up. In the regime of few tens to a hundred nanometer of single layer thickness, the dislocation pileup-based H-P model breaks down. This can be explained in terms of the limiting dislocation size criterion proposed by Cheng et al.