We have calculated the structural, electronic, mechanical, and optical properties of cubic and orthorhombic UO 2 , using GGA + U approximation along with spin-orbit coupling. A phase transition from cubic (Fm-3m) structure to orthorhombic structure (Pnma) occurs in UO 2 at higher pressure. The transition pressure is obtained as ∼47 GPa with a volume loss of 5.3%. The band-gap for orthorhombic structure increases as compared to the cubic structure. However, the application of pressure in cubic UO 2 causes a decrease in band-gap. UO 2 is found to be mechanically stable in both cubic and orthorhombic structures. The elastic constant increases with an increase in pressure in cubic UO 2. The inclusion of SOC causes elastic softening. The results obtained in this work are in excellent agreement with experiments. [Display omitted] • We used DFT+U method with spin orbit coupling (SOC) to study the properties of 3k AFM ground state of UO 2. • We found a structural phase transition at ∼47 GPa with a volume loss of 5.3%. • An indirect to direct band-gap transition has been observed at transition from cubic to orthorhombic structure. • UO 2 is mechanically stable in cubic as well as orthorhombic structure and elastic constants of cubic UO 2 increases with applied pressure.. • The electronegativity difference between U and O atom decreases with an increase in pressure in cubic UO 2. [ABSTRACT FROM AUTHOR]