In Iran about 24% of mortalities due to traffic accidents are brought about by motorcyclists . head injury is accounted as the main cause of this (65.7%-75%) fatality. Although helmet wearing can significantly decrease the risk of head injury , few motorcyclists make use of it. Planned Behavior Model is one of the important models which explains the major mechanism for accepting health behaivors.In the present study the Extended Model of Planned Behavior was applied in predicting helmet wearing among motorcyclist clerks in Yazd