Reinforced thermoplastic pipe (RTP) is a composite thermoplastic pipe, which is increasingly being used in oil and gas industry. In practical applications, RTPs inevitably experience bending during reeling process and offshore installation. The ovalization instability of RTP under pure bending was investigated. Several fundamental assumptions of RTP were proposed from the engineering application point of view. Then, based on nonlinear ring theory initially proposed by Kyriakides et al., the effect of transverse deformation through the thickness was introduced, and the ovalization growth of cross section during bending was studied according to nonlinear kinematics. The formulation was based on the principle of virtual work and was solved by a numerical solution. Inelastic material behavior of high density polyethylene (HDPE) was included, and a simplified method was proposed to simulate the behavior of fiber reinforced layer. A detailed Abaqus model was established using solid and truss elements to simulate the HDPE layer and reinforced fiber, respectively. The results obtained from the theoretical method were compared with Abaqus simulation results and test data of verification bending experiment and the results show excellent agreement. The proposed methods are helpful for RTP's engineering applications.