Research of ternary AIIIBIIIC2VI compounds (where A and B represent metal atom like Tl and Ga, In, respectively, C represents chalcogen atom) are part of the majority in the field of semiconductor research in Azerbaijan and throughout the world. These compounds have been studied intensively in recent years . Occurrence of the low-temperature phase transitions in these compounds according to the observations of soft modes, the anomalies in the dielectric spectra, the spontaneous polarization and birefringence anomalies, together with the highly pronounced layer structure and its possible modulation, make them an interesting subject for future fundamental research and practical applications, especially in optoelectronics. Physical properties of these materials have been studied intensively, including the IR and Raman active phonons at ambient conditions as well as at low temperature and high hydrostatic pressure. At the same time there are not many publications devoted to the lattice dynamics of these compounds (to our knowledge there exist not more than 4 publications in which the authors consider the lattice dynamics some of these compounds in the frame of the linear-chain model). TlGaSe2 crystallizes in a monoclinic system with the base-centered lattice and space group symmetry C2h6 at 300 K. The primitive cell contains 8 formula units. The crystal structure consists of layers composed of tetrahedral complexes Ga4Se10 linked together by the common atoms of selenium. Univalent thallium ions are in trigonal-prismatic voids between these complexes. Two layers within the unit cell are rotated relative to each other at 90o. In this work we present the results of the first-principle calculations of the lattice dynamics of TlGaSe2. For this purpose we performed the calculations by using open-source code ABINIT on the basis of the density functional perturbation theory within the plane wave pseudopotential approach. The basis of plane waves was truncated at electron kinetic energy of 40 Hartrees. Integration over the Brillouin zone was carried out using a 2x2x2 grid. The equilibrium structure was determined by minimizing the total energy with respect to the lattice constants and the internal structural parameters. Dynamics matrix describing the phonon spectrum throughout the Brillouin zone was obtained by Fourier interpolation using the ANADDB program of the ABINIT software package. Results on the frequencies of phonon modes in the centre of Brilloin zone and the dispersion of transverse shear acoustic branch of the phonon spectra agree well with experimental data on Raman scattering, IR reflectivity and ultrasound wave propagation in TlGaSe2. The calculated and experimental temperature dependencies of heat capacity are in a good agreement up to the room temperature. Along the layer, the low-frequency acoustic branch displays the bending wave behavior which is typical for the crystals having layered structure.