The research relevance. Nanoelectromechanical systems, being highly sensitive sensors with small dimensions and reliable in operation, are increasingly used in the oil and gas industry for monitoring various processes in oil production, from exploration to enhanced oil reco-very, as well as in well drilling, cleaning, fractionation and processing before decommissioning. One application example of nanoelectro-mechanical systems is seismic exploration. The use of nanoelectromechanical systems offers improved performance in addition to signifi-cant cost and time savings for a wide range of oil and gas industry technologies. With continuous monitoring capabilities, these technolo-gies can become the foundation of smart deposits. The main aim of the research is a construction of a mathematical model that most closely describes the sensitive element nonlinear dy-namics of a nanoelectromechanical sensor under an alternating load action. For this, it is necessary to take into account the most common kinematic hypotheses, scale effects using the modified couple stress theory of elasticity, the nonlinear relationship between stresses and strains, the material inhomogeneity, noise and thermal fields. And also to examine the complex nonlinear oscillations nature and identify patterns of transition from harmonic to chaotic. Objects: geometrically and physically nonlinear nanobeam, described by the kinematic model of the first approximation, which is affected by a uniformly distributed alternating transverse load with a harmonic component, the temperature field and additive external noise. Methods: variation methods, a second-order finite difference method for reducing the system of nonlinear partial differential equations to the Cauchy problem, the Newmark method for solving the Cauchy problem, the Birger method of variable elasticity parameters for solving a physically non-linear problem, the variation iteration method for obtaining an analytical solution of the two-dimensional heat equation. Results. The variation iterations method is used to obtain an analytical solution of thermal conductivity. An oscillations mathematical model for the sensitive element of the nanoelectromechanical sensor in the form of a size-dependent beam, on which a uniformly distributed transverse load with a harmonic component acts, is constructed. In addition to the variable load, the influence of the temperature field and additive external noise exposure were taken into account. The geometric nonlinearity is accepted according to Theodore von Karman theory (the relationship between deformations and displacements). To take into account the physical nonlinearity of the beam material, the deformation plasticity theory and the method of variable elasticity parameters are used. The motion equations of a mechanical system element, as well as the correspond-ing boundary and initial conditions, are derived from the Ostrogradsky-Hamilton principle based on a modified moment theory taking into ac-count the Euler-Bernoulli hypothesis. It was revealed that the temperature and noise fields reduce the load at which the transition to the chaotic state of the system occurs. The transition from harmonic to chaotic oscillations occurs according to Ruelle-Takens-Newhouse scenario. © 2020 Tomsk Polytechnic University, Publishing House.