The attitude dynamics equation of free-floating space robots subjected to gravitational gradient effect is investigated. A two-link space robot is employed to analyze nonlinear properties of the perturbed yaw motion of the system in depth, when the manipulator configuration is fixed. Its nonlinear dynamical behavior is described by phase plane plot and Poincaré section. It is shown that the perturbed motion is sensitive to orbital eccentricity. The system takes on periodic motion and tumbling in circular orbit, while additional quasi-periodic motion in elliptic orbit. Furthermore, these nonlinearities are quantitatively studied by means of bifurcation and power spectrum.