With the aim of increasing the $CoQ_{10}$ production in mass culture, the effect of aeration-agitation on the $CoQ_{10}$ production using Rhodobactor sphaeroides was investigated in a l-L bioreactor. The maximum $CoQ_{10}$ production was 1.69 mg/g of dry cell weight under conditions of 50 Lux, $30^{circ}C$, 300 rpm, and 5-vvm aeration. The $CoQ_{10}$ production was improved to produce 2.91 mg/g of dry cell weight under reduced conditions of agitation speed (200 rpm) and aeration rate (0.2 vvm). When R. sphaeroides was cultivated under more reduced DO levels during the exponential phase of the cell, the $CoQ_{10}$ production yield of 3.88-mg/g dry cell weight was the maximum obtained. Therefore, poorer conditions of aeration-agitation resulted in higher production of $CoQ_{10}$, and thus DO content was a crucial factor in the production of $CoQ_{10}$. Accordingly, it was necessary to control the DO concentration in order to enhance the $CoQ_{10}$ biosynthesis within a large-scale production.