Mass spectrometry is one of the most powerful techniques in chemistry. This technique includes a wide range of applications such as elemental analysis, surface analysis, measurement of molecular mass, molecular structure identification, evaluation and determination of the concentration of reactive gas phase components of in a sample. In the present work, The Matrix Assisted Laser Desorption Ionization (MALDI) mass spectrometry method was started-up in Iran for the first time. In MALDI mass spectrometry, a matrix for the protonation of sample and laser radiation for the vaporization and ionization of sample are used. The results of this section includes the recorded MALDI spectra of the amino acids which the ionic fragmente (MH + ) is observable in the spectra. In addition, the effect of the experimental conditions such as wavelength radiation, the kind of surface and the concentration of amino acids on the MALDI spectra were studied. It was found that the gold surface, the concentration about 400 µM of amino acid and IR radiation are the best optimized conditions for the MALDI mass spectrometry in our work. In the second part, ion attachment method were used for the ionization of species. Ion attachment mass spectrometry (IAMS) is one of the types of mass spectrometry soft ionization techniques which the fragmentation of sample molecules is low. Cations of samples are created from alkali salts by laser radiation directly on the sample so that the metal cations are attached the sample molecules. The mass spectra of proline and glycine were recorded by this method and the ions as [AA + 2M-H] + and [AA + M] + were observed where AA identifies amino acids and M alkali metal cation. In the third part of this section, the laser light was irradiated indirectly (from backside) on the sample plate and alkali ions were connected to the sample molecules and the desired species were observed. The recorded spectra by Indirect Laser Desorption Ionization (ILDI) method were better than those recorded by the direct radiation method. This method can be considered as a new technique, a simple, easy and effective way for the mass spectrometry of biological molecules. In computational section of thesis, the photoelectron spectra of biological molecules including of unsaturated, saturated steroids and fat-soluble vitamins in the range of 7-13 eV, were calculated. The calculated photoelectron spectrum of each compound was compared with its corresponding experimental spectrum reported in literature. It was observed that the assignment of ionization bands of steroid are very sensitive to the molecular structure so that a negligible change of composition and /or conformation, yields changes in the assignments of the ionization bands and creates dramatic changes of modes of action of steroid. It should be mentioned that although the most probable sites for ionization (below 11 eV) in the considered steroids in this work are p C=C and lone electron pairs of oxygen atoms of carbonyl and hydroxyl groups but, the energy order of ionization bands related to them are strongly depend on the geometrical structure of steroid. Finally, we found that the ionization of the selected steroids from the p electrons of C=O bond is not probable for the binding energy below 11 eV. In addition In other section of thesis, the ionization energies and valence photoelectron spectra of different vitamins including A, K, E and D were calculated using direct SAC-CI method. The present calculations finely simulated the shape of the experimental spectra and provided the detailed assignment of the spectra.