The quantum mechanical MMDO/3-Forces method has been applied as an entry to evaluate the vibrational spectra of N-hetrocycles with different ring size. The wavefunctions, dipole moment and equilibrium geometry for each molecule are calculated. The cartesian forces and dipole moments are determined for the molecule after displacing each cartesian coordinate from its equilibrium position by +0.01 A° and - 0.01 A°. Then cartesian force constant is obtained numerically as the difference between each two forces related to ±ve displacement of the same coordinate divided by the total displacement (0.02 A°). Thereafter the Cartesian force constant matrix is applied to compute the (3N-6) harmonic vibrational frequencies (in cm-1). The numerical dipole moment derivatives are used to obtain the IR absorption intensities and the value for the atomic partial participation APP in the motion for each normal mode obtained from the solution of the vibrational secular equation. The cartesian coordinates of the equilibrium geometryand the vibrational eigenvectors are used to plot the graphical representation for each fundamental vibration and draw the atomic displacement vectors. The vectors represent the amplitudes and directions of the atomic motions for each vibration.The graphical representation with the aid of APP values are employed to assign the vibrational modes in both the chemical and the group theoretical convention.The theoretical method adopted in this work minimizes the mathematical steps relative to other theoretical methods published in the literature. It provides the capability of using a personal computer with relatively small central memory, which in turn consumes a shorter computer running time. The vibrational spectra of nineteen molecules are calculated theoretically, most of them are heterocyclic compounds with one or more nitrogen atoms. They are divided into four groups, which are the six-, five-, four- and the three membered ring heterocycles. The obtained frequencies are in general quite satisfactory when compared with the available experimental and other theoretical data. The deviation percents are within the accepted range apart from few frequencies. This may be related to discrepancies between our and the experimental assignment, since there are many problems in achieving a full interpretation of complicated observed spectra of some molecules. The present results indicate that the chosen theoretical approach is generally suitable for getting a complete reliable assignment for all fundamental vibrations as well as making useful predictions of the spectra for unknown molecules.