Molecular dynamics (MD) is a form of computer simulation wherein atoms and molecules are allowed to interact for a period of time under known laws of physics, giving a view of the motion of the atoms. This method is used for analysis of many physical systems. In this thesis, MD is used for the simulations of two solid states phases of aspirin, the multi purposes medicine. Investigation of energy and structure of both phases are performed at 100 K, 123 K and 180 K at 1atm. The used force field was AMBER force field and the used software was DL_POLY 2.15. The aspirin molecules were modeled in rigid form. The time step for the simulations was 1.0 fs. The important result of these simulations is that the energy of form II is lower than that of form I. The structures of two solid state phases of aspirin were determined by radial distribution functions. The second part of this thesis is about the MD simulations of aspirin solutions (form I) in water and methanol. The results of these simulations are used in calculation of solvation energies and densities of all the solutions and also partial molar energies and partial molar volume in some particular range of concentration. The comparison of solvation energies of aspirin in methanol and water shows that aspirin dissolves easier in methanol than water. Also the results of configuration energies show that the configuration energies change linearly with change of concentration and partial molar energy is nearly constant. The solvation of two solution phases of aspirin is analyzed by radial distribution functions. In the end the results of MD calculations of aspirin solution in water is compared with the results of Gaussian calculations of gas phase of aspirin-water complexes.