The structural, electronic, bonding, and elastic properties of the low-temperature orthorhombic phase of NH 3 BH 3 are studied by means of first-principles total energy calculations based on the pseudopotential method. The calculated structural parameters of NH 3 BH 3 are found to be in good agreement with the experimental values. From the band structure calculations, the compound is found to be an indirect bandgap insulator with the bandgap of 5.65 eV (5.90 eV) with LDA(GGA) along the -Z direction. The Mulliken bond population and the charge density distributions are used to analyze the chemical bonding in NH 3 BH 3 . The study reveals that B-H bonds are more covalent than N-H bonds. The elastic constants are predicted for ambient as well as pressures up to 6 GPa, from which theoretical values of all the related mechanical properties such as bulk modulus, shear modulus, Young's modulus, Poisson's ratio, and anisotropy factors are calculated. It is found that NH 3 BH 3 is mechanically stable at ambient and also external pressures up to 6 GPa. As pressure increases all the calculated elastic moduli of NH 3 BH 3 increase, indicating that the compound becomes more stiffer and hard under pressure. From the ratio of shear modulus to bulk modulus (G/B), we conclude NH 3 BH 3 to be ductile in nature, and the ductility increases with pressure. The present results confirm the experimentally observed less plastic nature of the low-temperature phase of NH 3 BH 3 .