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Hard coatings have been widely applied to enhance the tribological performance of mechanical components. However, it was predicted that thin hard coatings may have a weakening effect of reducing the coating/substrate systems resistance to plastic yielding compared with an uncoated substrate. An analytical simulation is utilized in this paper to investigate the origination of weakening effect. The functions of coating thickness as well as mechanical properties of coating and substrate material on the weakening effect are explored theoretically. Partial-unloading spherical nanoindentation tests are performed on TiN coated single crystalline silicon to acquire the stress-strain curve and compared with the uncoated case. The experimental results are in consistence with the analytical solutions, demonstrating the presence of weakening effect. A novel criterion based on weakening effect is established to direct the design of thin hard coating systems.