Molecular dynamics simulations have been performed to study the dynamic behaviors of single crystal aragonite under indentation, tension, and compression. The elastic modulus and hardness of single crystalline aragonite measured in our simulations are found in good agreement with experimentally measured values. Our simulation results show that the mechanical properties of aragonite crystal, including the elastic modulus, hardness, strength, and toughness, strongly depend on the crystallographic orientations and loading conditions. We have identified that this dependence is resulted from different deformation mechanisms, i.e., phase transformation, amorphous phase formation, dislocation, and twining. This work is an attempt to identify the deformation mechanisms in aragonite and to establish the relationship between the dominant deformation mechanisms and its crystallographic orientations and loading conditions.