A small jar fermentor was developed in order to investigate the effect of oxygen supply on hydrocarbon fermentation. Several indices to oxygen transfer were examined with this small jar fermentor. Conditions for suitable oxygen supply were examined in l-glutamic acid fermentation from hydrocarbon by use of shaking flasks and these small jar fermentors. The data indicated that the rate of oxygen transfer ought to be more than 14.3 × 10−7 mole/ml·min in order to obtain satisfactory results. The coefficient of oxygen transfer rate (KLa/H) decreased as the fermentation went on, so the supply of oxygen enriched gas mixture was effective to increase the production of l-glutamic acid.