<p>For the first time, this article provides essential knowledge about the required inner radius-to-pipe radius ratio for selecting the optimal graphene sheet distribution pattern. This selection aims to improve resistance against divergence while enhancing both linear and nonlinear natural frequencies for oscillating fluid-filled composite pipes reinforced with graphene sheets (GRC pipes) exposed to a thermal environment. In this context, the equations of motion are presented based on the Euler–Bernoulli beam model, taking into account geometric nonlinearity through von Karman nonlinear strains. The flow is assumed to be fully developed, making the plug flow model applicable. The refined Halpin–Tsai rules are used to specify the mechanical properties of GRC. The nonlinear discretized equations of motion, obtained through the Galerkin projection method, are analyzed using the method of multiple scales to derive the linear and nonlinear natural frequencies, as well as the associated nonlinearity coefficient. The effects of inner radius-to-pipe radius ratio, in conjunction with the graphene sheet distribution pattern, layer arrangement, flow speed, and environmental temperature on the outcomes are illustrated. For the first time, it is demonstrated that the radius ratio significantly affects the optimal distribution pattern for a GRC pipe, leading to improved performance against divergence, along with enhanced linear and nonlinear natural frequencies. The relationship between these two factors is critical. Thus, this article clarifies the impacts of the radius ratio and the graphene sheet distribution pattern on the linear and nonlinear free oscillation characteristics of fluid-filled GRC pipes, offering valuable insights for engineering designers. Additionally, the significance of layer arrangement for fluid-filled GRC pipes exposed to a thermal environment is highlighted.</p>