In dynamic positioning systems, the horizontal plane motions, including surge, sway and yaw, are commonly taken into consideration. However, the vertical plane motions, especially roll and pitch, which are induced by thruster force may have influence on positioning of marine constructions as well. Horizontal plane motion control with roll and pitch control might be a solution to this problem. Roll and pitch velocity and acceleration are feedback signals to generate the control forces in the horizontal plane. The effect of the additional roll and pitch motion control law on the positioning accuracy of the horizontal plane remains uncertain. In this paper the analysis of roll and pitch motion of a dynamic positioned semi-submersible with roll and pitch motion control is made. The mechanism of the effect under the roll and pitch control law is presented in the time domain. It is shown that the effect is determined by the attitude of the semi-submersible, as well as the horizontal control force at that moment. The analysis of the influence on positioning accuracy is conducted based on the statistical analysis of the time-domain motions.