Raman study of 3-methyl-4-nitropyridine-N-oxide single crystal (3M4NPO) has been performed at 78 K in the range 10-3500 cm-'. The symmetry analysis of the vibrational modes of 3M4NPO is given. The assignments are presented for internal and external modes. The results of the Raman spectra exhibit the spectroscopic proofs of hydrogen bonds in the crystal. THE 3-METHYL-4-NITROPYRIDINE-N-OXIDE (3M4NPO) Crystal has high nonlinear Optical coefficients, high optical damage threshold and excellent phase matching properties [l-7]. Its crystal structure has been determined by SHIRO et al. [8] and BAERT et al. [9]. The only incomplete result concerning vibrational spectroscopy at room temperature was done by BAERT et al. [9] who were interested in the C-H stretching vibrations, by which the positions of hydrogen atoms can be determined. In this work, the polarized Raman spectra of 3M4NPO single crystal were recorded at room temperature and 78 K, and the vibrational modes are discussed in detail. EXPERIMENTAL The preparation of 3M4NPO consisted of two steps: firstly, 3-methyl pyridine was oxidized by 30% Hz02 at 70Β°C and then, was treated by concentrated nitric acid at 100Β°C. The reaction products in every step were refined by means of extraction separation, depression distillation and recrystallization. The single crystals were grown in saturated acetone solution by evaporating the solvent and large pale yellow crystals were obtained. The sample used for Raman measurements was made in the form of rectangular platelet of a x b x c = 1 x 5 x 4 mm3 and the faces were polished. INNOVA 70-5 argon ion laser radiation (514.5 nm, 80mW) was used to excite Raman scattering, and scattered light was analysed with Spex-1403 double monochromator. The sample was mounted inside the vacuum chamber of a LT-3-110 refrigerator. Raman spectra were recorded at room temperature and 78 K from 10 to 3500 cm-' with scanning steps of 2 cm-' in the scattering configurations b(cc)a, b(ab)a, c(ac)a and b(cb)c, corresponding to A, B,, B2, and B3 modes, respectively.