Density functional theory and two-configuration self-consistent-field calculations were carried out to investigate the unimolecular isomerizations of cyclopropane. The calculated structural parameters and vibrational frequencies of cyclopropane are in good agreement with the measured values. Obtained structures and relative energies of the transition states for the geometrical (cis-trans) and optical isomerizations agree with previous calculations. A transition state for the structural isomerization cyclopropane --> propylene was located, and its energy level and vibrational frequencies were calculated with uB3LYP/cc-pVDZ method. IRC calculations using the same method and basis set show that this transition state is connected to cyclopropane and propylene without an intermediate. The wave function of the transition state contains a contribution of ionic and biradical terms. The calculated activation energy and preexponential factor for k(infinity) are in very good agreement with the experimental values. RRKM calculations of the first-order rate constant for the structural isomerization were carried out over a wide range of pressures and temperatures.