Slow scan CT (SSCT) has been utilized for radiation treatment planning based on the assumption that coverage of full tumor motion during scanning time would yield a reasonable output in determining internal target volume (ITV) margin. However, inconsistency in projection data due to the motion would result severe distortion in reconstructed image and therefore, careful precaution must be taken. In this study, we have simulated and evaluated the effect of tumor motion on reconstructed image of SSCT with different respiratory durations. Projection data was generated through ray tracing forward projection method assuming that tumor sphere oscillates vertically with sine function at single respiratory duration. Measurement of projection data was taken by full coverage of respiratory duration from single to ten. Evaluation was done with calculating percentage radial deviation at the trajectory of tumor motion. Results have shown that at least three times of duration coverage during the scan time is necessary to represent ITV close to that in ideal. The findings of this study have shown that careful precaution on motion coverage must be taken before acquiring appropriate ITV margin of SSCT.