Problem. Torsional vibrations of the internal combustion engine crankshaft are the result of uneven torque due to gas pressure in the cylinders, as well as due to inertial forces of engine moving parts. To analyze torsional vibrations, it is necessary to have an adequate mathematical model of the engine mechanical system that takes its dynamic characteristics into account. Goal. Developing methodology for determining the mass-inertial parameters of connecting rod mechanisms in the internal combustion engines and building dynamic models based on the Openmodelica environment. Developing the Modelica language algorithm for calculating natural frequencies and forms of the dynamic model oscillations. Methodology. The elastic and inertial parameters of the dynamic scheme elements are determined by their energy equivalence to the mechanical system. The construction of a dynamic and mathematical model is implemented using the Modelica programming language. Results. Based on the proposed methodology, dynamic models of straight and V internal combustion engines were built in the Openmodelica environment. Calculation of system oscillation natural frequencies and forms was carried out. The results obtained were compared with data from other sources and analyzed from the physical feasibility point of view. Originality. The proposed methodology for determining the mass-inertial parameters of the connecting rod mechanisms in the internal combustion engines and the way to build dynamic models based on them has been shown. An algorithm has been developed in Modelica language, and natural oscillation frequencies and forms were calculated. Practical value. The developed models can be used as a part of the higher-level dynamic machine models, as well as for determining the parameters of engine crankshaft torsional vibrations.