This work investigates the structural and electronic properties of pure and Sr-doped ZnO using first principles density functional calculations (DFT). The calculations were carried out using GGA-BLYP functional. This functional underestimates the band gap value in semiconductors but does not affect the accuracy of the related properties of the crystals. The Sr-doping caused increase in lattice volume and slight distortions at the unit cell parameters in a wurtzite structure. The doping process presented increase in the band-gap energy Eg at low percentages 25%, 37.5% and 50% with indirect bang gap and direct band gap at high percentages 62.5%, 75%, 87.5% and 100%.which we can called it wide indirect band gap. These results can be use as a foundation for more in depth calculations which can be used on optical and Photo-catalytic applications.