This study investigated the impact of five organic loading rates (OLRs) ranging from 16 kg COD/m3-d to 32 kg COD/m3-d on the performance of a continuous stirred tank reactor (CSTR) for biological hydrogen production, with molasses as substrate. Hydraulic Retention Time (HRT) was kept at 6 hours and the temperature 35°C. Oxidation-reduction potential (ORP) varied from -328 mV to -419 mV. The ethanol-type fermentation has been conducted during the process. A positive statistical correlation was observed between OLR and biogas yield; however, it became negative for the OLR of 32 kg COD/m3-d (Fig1). The system maintained the highest average biogas and hydrogen yield of 14.66 L/d and 5.17 L/d at OLR of 28 kg COD/m3-d. Compared with the first stage (16 kg COD/m3-d), ethanol and butyric concentration of the last stage (32 kg COD/m3-d) increased about 2.01 and 3.79 times, respectively; Acetic acid concentration surged around 200 mg/L; Prop ionic acid concentration dropped slightly; Valeric acid concentration, which took a small portion in the volatile fatty acid (VFA), kept low. When OLR was kept at 28 kg COD/m3-d, the average ethanol to acetic acid ratio is 3.90.Thus proved that the optimal OLR for hydrogen production under experimental condition is 28 kg COD/m3-d.