NbSi2 is an attractive material for high temperature applications due to its high melting point, low density and good oxidation resistance. The high-temperature strength of NbSi2 is expected to be further improved by incorporation with Nb5Si3, which performs a high creep resistance and strength at high temperature due to its complex crystal structure. In this paper, directionally solidified NbSi2/Nb5Si3 in-situ composites have been prepared using an optical floating zone method. Scanning Electron Microscopes (SEM) and X-ray diffraction (XRD) have been used to investigate the phase constitution and microstructure. The orientation relationship between Nb5Si3 and NbSi2 is investigated by transmission electron microscopy (TEM). High-temperature properties of alloys are tested by compression at the strain rate of 1 x 10-(4)/s at 1673K and 1773K. It was found that high temperature strength and phase constitution of directionally solidified alloys depended on the addition of Mo.