BACKGROUND: This study aimed to empirically investigate the determinants on oxygen free radical degree which causes biological damage focusing on smoking habit and health behaviors in daily life. METHODS: In this cross-sectional study, the subjects were adult males who submitted free radical tests among the examinees of the comprehensive medical testings at an university hospital in Suwon from 2008 to 2010. Total 497 samples were used in analyses for clarifying the relations of free radical degree to age, smoking, obesity, and other health behaviors. RESULTS: First, the degree of oxygen free radical increases by 0.164 as the age group increases (P<0.05). Second, the degree of oxygen free radical increases by 0.547 as the smoking habit increases (P<0.001). Third, the degree of oxygen free radical increases by 0.087 as the exposure to hazardous factors in workplace increases (P<0.001). CONCLUSIONS: The findings of the study imply that the smoking habit has more great influence on the increase of oxygen free radical in body than any other health behaviors. The causality of free radical to smoking and health behavior was not identified due to the limit of the cross-sectional study so it needs to be studied prospectively in the future.