Abstract: Effects of strain on electronic structure and optical properties of wurtzite GaN are studied by using generalized gradient approximation (GGA+U) under first-principles density functional theory (DFT). It shows that the bandgap decreases with increase of strain. The decrease of band gap is small as compressive strain is less than 3%. Strain has an effect on dielectric function imaginary part. With increase of strain, static dielectric constant increases and absorption coefficient decreases.