Density functional theory calculations have been carried out to calculate the electronic structure and optical properties of host and Cs doped KMgCl 3 . All the calculations were performed by using the Tran and Blaha modified Becke-Johnson potential (TB-mBJ) in order to accurately predict the band gap and the optical spectra. The investigated compound is found to be an insulator with direct band gap of 4.7, 6.9 eV using generalized gradient approximation (GGA), and TB-mBJ functionals respectively. The calculated refractive index shows the optical isotropy of this compound in the low energy region, though the structure is anisotropic. From our theoretical calculations we predict KMgCl 3 doped with Cs to be a better cross luminescence material compared to host compound, where additional Cs states are present below the valence band of the compound. Detailed discussion is presented in the manuscript. © 2019 IOP Publishing Ltd.