The relationship between the morphological character of Pseudomonas denitrificans and vitamin B-12 synthesis based on real-time capacitance measurement and online specific oxygen consumption rate (Q (O2)) control was established for enhancing vitamin B-12 production. Results demonstrated that the threshold Q (O2) value lower than 2.0 mmol/gDCW/l would greatly stimulate the state transfer from the cell number growth phase to the cell elongation phase and promote rapid vitamin B-12 biosynthesis, while the vitamin B-12 biosynthesis rate could also be inhibited when the rate of cell's length-to-width ratio (ratio-LW) was higher than 10:1. Furthermore, the optimal morphology controlling strategy was achieved based on online Q (O2) control, which increases the appropriate active cell numbers at the former phase, and then control the elongation of ratio-LW no more than 10:1 at the vitamin B-12 biosynthesis phase. The maximal vitamin B-12 production reached 239.7 mg/l at 168 h, which was improved by 14.7 % compared with the control (208 mg/l). This online controlling strategy would be effectively applied for improving industrial vitamin B-12 fermentation.