The structural stability and mechanical properties of *α*-Nb_{5}Si_{3} alloyed with Ti, Cr, Al and B were investigated using first- principles methods based on density functional theory (DFT) by comparing the formation energy, valence electron concentrations, elastic constants, the shear modulus/bulk modulus ratio, and the Peierls stress. The results show that the structures of the *α*-Nb_{5}Si_{3} alloys retain the stable D8_{1} structure, in which the alloying elements Ti, Cr, Al and B prefer to occupy the Nb^{4c}, Nb^{4c}, Si^{4a} and Si^{8h} sites of *α*-Nb_{5}Si_{3}, respectively. The addition of Ti, Al and B increase the brittleness of D8_{1} structured *α*-Nb_{5}Si_{3}, while Cr addition is beneficial to the toughness of *α*-Nb_{5}Si_{3}. Moreover, the influence of the alloying elements on the ductility/brittleness of *α*-Nb_{5}Si_{3} was investigated based on analysis of the electronic structure, density of states and Mulliken population. The increased brittleness of *α*-Nb_{5}Si_{3} by the addition of Ti, Al and B can be attributed to enhanced orientation of the covalent bonds, whereas Cr addition weakens the number and strength of covalent bonds and more anti-bonding states are occupied, thus improving the toughness.