Lead-free perovskites have drawn much attention of researchers in the field of electronics and photovoltaics due to the toxicity issue of the lead halide perovskites. The methylammonium tin iodide CH 3 NH 3 SnI 3 amongst others has become a viable alternative due to its eco-friendliness, as well as narrower bandgap and its wider visible absorption spectrum. In this study different theoretical approaches were employed in investigating the structural, electronic and thermodynamic properties of the orthorhombic phase (O-phase) of the CH 3 NH 3 SnI 3 perovskite. By using the first-principle calculations with the density functional theory, a direct bandgap was determined at gamma symmetry points with three exchangecorrelation functionals: PBE 1.12 eV, PBEsol 0.98 eV, and LDA 0.46 eV. Based on the comparison of lattice constants and bandgaps with the experimental values, the best performance resulted from PBE. The decomposition of the CH 3 NH 3 SnI 3 perovskite into solid state products, CH 3 NH 3 I and SnI 2 , was considered; the enthalpy of the reaction Δ r H° (0 K) = 37 kJ mol −1 and enthalpy of formation of the O-phase perovskite Δ f H° (CH 3 NH 3 SnI 3 , 0 K) = − 390 kJ mol −1 were evaluated, indicating the stability of the O-phase CH 3 NH 3 SnI 3 at low temperature, in agreement with experimental findings.