This project is on First principle calculations of the structural, electronic and mechanical properties of ZnS1xSex semiconductor alloy. The structural, electronic and mechanical properties of ZnS1-xSex semiconductor alloy at the ground state was investigated by first-principle calculations based on projector-augmented wave (PAW) density functional theory (DFT) within the generalized gradient approximation (GGA) and Perdew-Burke-Ernzerhof (PBE) approach over the whole composition range ( 0 x 1). The calculated parameters for the structural properties are the lattice constants, bulk modulus and its pressure derivatives. The lattice parameter increases from 5.44Å to 5.75Å while the bulk modulus decreases from 74.6GPa to 58.6GPa. For the electronic properties, the band gaps are estimated for the binary compounds and the alloys. The band structures and the density of states are also presented to explain the individual contribution of each atom to the general electronic behaviour of the material. The calculated band gap indicates a direct band and decreases from 2.07eV to 1.11eV. The three independent elastic constants C11, C12, and C44 are also calculated to explain the mechanical stability of the alloy. Also, computed and presented are the shear modulus, anisotropy, young’s modulus, poisson’s ratio, and hardness of the material. The bulk to shear ratio, B/G are also considered important in explaining the ductile/brittle behaviour of the material, and are also carefully computed and presented. In the composition range, the values of B/G are greater than the critical value 1.75 and ranges up to 2.05 revealing the ductility of the material. Our results in general are consistent with experiments and other theoretical calculations as we allow for comparison with available data.