A review and evaluation аthe completeness of the experimental study on the thermodynamic properties (TDP) of n-octadecane in various ranges of state parameters has been carried out. Data on the thermodynamic properties of domestic and foreign authors, obtained both in the last century and in recent years, were analyzed. As a result of a critical analysis, the most representative and reliable ones were selected from them. The analysis conducted revealed a problem associated with insufficient experimental study of the state surface of n-octadecane, in particular, in the supercritical and critical regions. In order to increase the numerical stability and improve the predictive capabilities of the developed equation of state, the authors decided to include the calculated TDP values in the array of processed experimental data. The calculated data were obtained by various proven methods within the theory of thermodynamic similarity and compensated for the data deficit in unexplored areas of state parameters. The resulting hybrid data array formed the basis for developing the fundamental equation of state (FES) of n-octadecane. To develop the FES, a modern heuristic algorithm was used that implements the random search method. The resulting equation reliably describes all TDP of n-octadecane in the temperature range from the triple point to 700 K at pressures up to 100 MPa. The equation of state is expressed in terms of one of the thermodynamic potentials - the Helmholtz free energy. The configuration part, describing the properties of a real fluid, contains 15 terms, of which five are polynomial, five are exponential, and five are Gauss terms for describing the properties in a wide vicinity of the critical point. The new equation of state satisfies the classical conditions of the critical point and physically correctly describes the behavior of the derivatives of the thermodynamic potential. The average absolute deviations (AAD) of the calculated values of TDP according to the FES from experimental data have the following values: saturation vapor pressure – ±3.545%, density of the saturated liquid – ±1.056%, isobaric heat capacity of the liquid phase at the saturation line – ±1.315%; sound speed in liquid phase – ± 0.4%, liquid phase density – ±0.4%, gas phase density – ±0.9%.