A determination method for elastic-plastic material constants that obey the power-law hardening rule, from a couple of indentation tests, is proposed. Π function indicates the relation between reduced Young's modulus E, load curvature C, representative stress σ, and indenter apex angles φ. The function is determined by Π theory and two solutions for a sharp indenter penetration, the elastic solution and the rigid/perfectly plastic solution. With Π function and another function which relates representative strain ε, to indenter apex angle, two or more experiments lead points in plastic region respectively, and the stress-strain curve is identified.