Pseudorotation (PR) of cyclic molecules is discussed. It is shown that PR can be considered as a special type of internal rotation with pseudorotational moment of inertia (PMI) as an important characteristic. The dependence of the PMI of a molecule upon the phase angle of PR and the nuclear trajectories is discussed. A method is proposed for the calculation of PMI from geometrical parameters of molecules. The moment of inertia of internal rotation for 1,2-dichloroethane in gauche conformation is calculated in order to test the proposed method. The obtained value is in good agreement with the values given by the Fitter method. Pseudorotational moments of inertia of cyclopentane and its derivatives are calculated. Calculated values are compared with those adjusted to fit experimental entropy. The dependence of the PMI of cyclopentane derivatives upon their molecular mass is discussed.