In the last decade, the traditional conveyor belt system has partly switched to a cellular manufacturing system in Japanese manufacturing industry. The new “cell” system has brought with both economic and environmental successes to leading companies. Although the new system stimulates the motivational aspects of workers by increasing job complexity, complex jobs are expected to place higher mental demands on workers with Type A personalities, who respond with adverse cardiovascular reactions to stressful situation. This study was designed to test an empirical hypothesis: the new “cell” system characterized by job complexity will result in greater perceived workload and heightened sympathetic arousal among participants rated high on the type A behavior pattern. Fourteen subjects (seven classified as Type A and seven classified as Type B) performed 90-minute repetitions of a simple assembly task in two different manufacturing systems; a conventional “conveyor” method and a new “cell” method. The results from the NASA Task Load Index and spectral analysis of heart rate variability presented evidence for the mediating role of the type A behavior pattern in the relationship between manufacturing system design and mental workload placed on participants. The findings imply that the type A behavior pattern of individuals should be assessed properly in the workplace and considered in evaluations of job design.