Quantitative data of mechanical properties such as hardness, H, and elastic modulus, E, are required for the constituent phases of an alloy in the process of alloy design. To meet the needs, the evaluation technique of H and E of phases in composites through nanoindentation tests is proposed. Moreover, H and E of Nb solid solution (NbSS) and niobium silicide (Nb5Si3) phases in Nb-base in-situ composites were characterized by the proposed method. To clarify the quantitative relationship between the nanohardness, Hn, and the micro-Vickers hardness, Hv, nanoindentation tests were carried out on the Hv Standard blocks with Hv100, 500, 700, 900 and 1600, under a wide range of applied loads from 0.1 to 40 mN. As a result, it was clarified that Hv and Hn are linearly related under each applied load. Therefore, Hv could be estimated from Hn by applying the linear relation. It was also confirmed that the elastic modulus is almost independent of the applied loads. Therefore, the elastic modulus, E, could also be directly estimated by nanoindentation tests with Poisson’s ratios of tested materials. Hv and E of NbSS and Nb5Si3 in the Nb-base composites determined by the method show good agreement with the reported values for both phases. Accordingly, it is possible to conclude that the proposed method is useful to quantitatively evaluate the hardness and elastic modulus of constituent phases in a composite.