<p>In this study, we adopted a 16-atom simple cubic supercell based on the special quasi random structure approach of Zunger et al. to investigate the structural, electronic and optical properties of InP_xAs_ySb_1-x–y quaternary alloys in zinc blende phase. The calculations were performed using the full potential linearized augmented plane wave (FP-LAPW) method within density functional theory (DFT). We employed the local intensity (LDA) and generalized gradient approximation of Wu and Cohen (GGA-WC) to calculate the structural parameters. Both schemes (EV-GGA) and (TB-mBJ) were used to describe the optoelectronic properties. Our calculations show that the lattice constant value of InP_xAs_ySb_1-s-y quaternary alloys decreases almost linearly with increasing P concentration. Electronic results showed that InP_xAs_ySb_1-s-y quaternary alloys exhibit a direct band gap semiconductor for all P concentrations. Furthermore, we observed that the direct band gap of quaternaries increases linearly with P concentrations. The optical properties of InP_xAs_ySb_1-s-y quaternaries, including the dielectric function, optical conductivity, absorption coefficient and the complex refractive index were predicted and discussed in detail. The obtained results make these compounds very promising for optical telecommunications.</p>