Inspired by the designs of underwater gliders, hybrid autonomous underwater vehicles (AUVs) have emerged recently, which use internal actuators instead of control surfaces to control the heading angle and depth of the vehicles. In this paper, we focus on controlling the heading angle of a REMUS AUV by using an internal moving mass. We derive a nonlinear dynamical model of the vehicle with hydrodynamic forces and coupling between the vehicle and the internal moving mass. The model is used to study the stability of the horizontal-plane motion of the vehicle and to design a linear feedback law to stabilize its heading angle around a desired direction. Simulation results demonstrate that a controlled internal moving mass is able to fulfill the purpose of heading control.