Cubic perovskite oxides RbSbO3 and CsSbO3 have been investigated for structural stability, electronic results, elastic, mechanical stability and thermodynamic results by most accurate density functional theory (DFT). The optimization has been completed using Local density approximation (LDA) and Generalized gradient approximation (GGA) within the scheme of Perdew, Burke and Ernzrhof (PBE). The ground state optimized results present minimum energy within GGA for both compounds. Band structure and density of state results both present the metallic nature for these compounds. The mechanical properties like Young's modulus, Bulk modulus etc. have been deduced from elastic values. RbSbO3 was found to have more resistance to compression as compared to strength CsSbO3. Both the materials were found to have brittle nature from Poisson's ratio (υ), Cauchy's pressure (C12-C44) and Pugh ratio (B/G) criteria. The melting temperature was calculated to be 2148 ± 300 K, 1746 ± 300 K, respectively for RbSbO3 and CsSbO3. Pressure and temperature variation has been used for calculation of thermodynamic parameters within quasi-harmonic Debye approximation. The nature of Bulk modulus, cell volume, specific heat capacity and thermal expansion has been computed in the temperature range of 0 K-900 K and pressure varied from 0 GPa to 15 GPa.