Abstract: The spinning principle of rotor spun composite yarns was discussed and different kinds of rotor spun composite yarns with different appearance and structures were produced by combining staple fibers with filament yarns under varying filament over-feed ratio on a modified open-end rotor spinning frame.The effect of filament over-feed ratio on the appearance and structure of composite yarns have been studied.Results showed that filament over-feed ratio has great influence on the filament geometric position and helix trajectory in composite yarns.