Three models, JKR (Johnson, Kendall and Roberts), DMT (Derjaguin, Muller, and Toporov) and MD (Maugis-Dugdale), are compared with the Hertz model in dealing with nano-contact problems. It has been shown that both the dimensionless load parameter, (P) over bar = P/(piDeltagammaRy), and the transition parameter have significant influences on the contact area at micro/nano-scale and should not be ignored in nano-indentation tests.