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The purpose of this paper is to investigate the effect of gravity gradient torques on attitude dynamics of on-orbit free-floating space robots. Nonlinear dynamics of the free-floating system subject to gravity gradient effect is derived by Lagrange equation. Firstly, the effect of nonlinear disturbances caused by gravity gradient torques on the motion free manipulator joints of free-floating space robot is analyzed. Secondly, when manipulator configuration is required to be fixed, the stability of pitch motion of the free-floating system in circular orbit is investigated by using phase phane portrait. Finally, the calculated errors of velocities of manipulator end with and without gravity gradient torques are developed. Disturbances to pose of the controlled manipulator end are talk about. Results show that the manipulator joints are significantly disturbed when time span is about an orbit period or longer, so that it cannot be neglected. Moreover, these disturbances are mainly determined by the eccentricity of orbit. The system with fixed configuration vibrates or tumbles along pitch axis. If initial angular rates are properly chosen, such system can avoid tumbling. For long time manipulation, large control errors are induced if the gravitational gradient effect is neglected. [ABSTRACT FROM AUTHOR]