It was concluded that there was sufficient evidence about a possible link between long working hours particularly exceeding 50 a week and the risk of significant health outcomes, including cardiovascular disease from literature review by Spurgeon et al.. This study was conducted to find out the single effect of regular overtime work on the cardiovascular functions through objective biological indices such as blood pressure or heart rate variability. We conducted a field survey of 238 male engineers who were working at the department of research & development of three electronics manufacturing companies in S. Korea. The field survey consisted of (1) self-report questionnaire (working hours and health conditions, and fatigue) and (2) measurements of blood pressure and heart rate variability. By multivariate analyses we could show the relationship between overtime work and some cardiovascular functions after controlling the effects of major confounders such as age and sleeping hours, which were pointed out by Iwasaki et al. (1998) and Sasaki et al. (1999). Especially, low frequency component (power in the low frequency range, 0.04-0.15 Hz) of the heart rate variability during work might be used as early objective biological indices for chronic effect of regular overtime work on cardiovascular functions. However, we should confirm those effect through the well-designed prospective study.