We propose a new theory with the potential for measuring the elastoplastic properties of compliant and soft materials using one sharp indentation test. The method makes use of the substrate effect, which is usually intended to be avoided during indentation tests. For indentation on a compliant and soft specimen of finite thickness bonded to a stiff and hard testing platform (or a compliant/soft thin film deposited on a stiff/hard substrate), the presence of the substrate significantly enhances the loading curvature which, theoretically, enables the determination of the material power-law elastic-plastic properties by using just one conical indentation test. Extensive finite element simulations are carried out to correlate the indentation characteristics with material properties. Based on these relationships, an effective reverse analysis algorithm is established to extract the material elastoplastic properties. By utilizing the substrate effect, the new technique has the potential to identify plastic materials with indistinguishable indentation behaviors in bulk forms. The error sensitivity and uniqueness of the solution are carefully investigated. Validity and application range of the proposed theory are discussed. In the limit where the substrate is taken to be rigid, the fundamental research is one of the first steps toward understanding the substrate effect during indentation on thin films deposited on deformable substrates.