In order to understand the deformation and fracture behavior of Nb-Si alloys, in-situ observation was conducted during bending of small specimens at room and high temperatures. Nb-Si alloy ingots containing 18.1 at.%Si, 1.5 at.%Zr and 100 ppmMg were prepared by arc melting, followed by uni-axial solidification in an optical floating zone apparatus and a heat-treatment to obtain Nb/Nb5Si3 two-phase microstructure. Chevron-notched specimens with a dimension of 1x2x10mm were used for in-situ observation of bending tests under a confocal laser scanning microscope (CLSM) at room temperature and at 1140°C. At room temperature the Nb-Si alloy shows a fracture toughness of 8 MPa m1/2 and the crack propagation velocity seems to be not uniform, presumably due to the ductile Nb. At 1140°C the toughness of the alloy was about 20 MPa m1/2 and slower plastic deformation prior to the cracking was observed. The SEM observation of crack surfaces revealed that plastic deformation of Nb enhances the toughness of the alloy.