Phosphorus (P) is widely used as n-type dopant for silicon (Si) to form the emitter layer in wafer-based silicon solar cells. The main purpose of this work is to investigate the influence of P doping on the structural and mechanical properties of silicon. CASTEP program, which uses the density functional theory (DFT), with a plane-wave basis, is used to study the structural, electronic, and mechanical properties of undoped and P-doped Si (Si1-xPx for 0.0001 <= x <= 0.05). The density of states (DOS), band structure, elastic constants, bulk modulus (B), Young's modulus (E), Shear modulus (G), and Poisson's ratio (v) were all calculated. It is found that brittleness of Si increased by P doping.