The structural,elastic,mechanical,electronic,and optical properties of KPaO_3 and RbPaO_3 compounds are investigated from first-principles calculations by using the WIEN2 k code in the frame of local density approximation(LDA) and generalized gradient approximation(GGA).The calculated ground state quantities,such as lattice constant(α_0),ground state energy(E),bulk modulus(S),and their pressure derivative(B_p) are in reasonable agreement with the present analytical and previous theoretical results and available experimental data.Based on several elastic and mechanical parameters,the structural stability,hardness,stiffness and the brittle and ductile behaviors are discussed,which reveal that protactiniumbased oxide series of perovskites is mechanically stable and possesses weak resistance to shear deformation compared with resistance to unidirectional compression while flexible and covalent behaviors are dominated in them.The analysis of band profile through Trans-Blaha modified Becke-Johnson(TB-mBJ) potential highlights the underestimation of bandgap with traditional density functional theory(DFT) approximation.Specific contribution of electronic states is investigated by means of total and partial density of states and it can be evaluated that both compounds are(Γ-Γ) direct bandgap semiconductors.All fundamental optical properties are analyzed while attention is paid to absorption and reflection spectra to explore extensive absorptions and reflections of these compounds in high frequency regions.The present method represents an influential approach to calculating the whole set of elastic,mechanical,and opto-electronic parameters,which would conduce to the understanding of various physical phenomena and empower the device engineers to implement these materials in flexible opto-electronic applications. The structural, elastic, mechanical, electronic, and optical properties of KPaO_3 and RbPaO_3 compounds are investigated from first-principles calculations by using the WIEN2 k code in the frame of local density approximation (LDA) and generalized gradient approximation (GGA). The calculated ground state quantities, such as lattice constant (α_0), ground state energy (E), bulk modulus (S), and their pressure derivatives (B_p) are in reasonable agreement with the present analytical and theoretical results and available experimental data.Based on several elastic and mechanical parameters, the structural stability, hardness, and the brittle and ductile behaviors are discussed, which reveal that protactiniumbased oxide series of perovskites is mechanically stable and possesses weak resistance to shear deformation compared with resistance to unidirectional compression while flexible and covalent behaviors are dominated in them. The analysis of band profile through Trans-Blaha modified Becke- Johnson (TB-mBJ) potential highlights the underestimation of bandgap with traditional density functional theory (DFT) approximation.Specific contribution of electronic states is investigated by means of total and partial density of states and it can be evaluated that both compounds are (Γ- Γ) direct bandgap semiconductors. All fundamental optical properties are analyzed while attention is paid to absorption and reflection spectra to explore extensive absorptions and reflections of these compounds in high frequency regions. The present method represents an influential approach to calculating the whole set of elastic, mechanical, and opto-electronic parameters, which would would be to the understanding of various physical phenomena and empower the device engineers to implement these materials in flexible opto-electronic applications.