The effects of hematite (Fe2O3) and nickel oxide (NiO) nanoparticles (NP) on biohydrogen production were investigated using dairy wastewater in batch test. The results showed that, unlike the NiO NP, the maximum hydrogen yield (HY) (16.75 mmol/g COD) and specific hydrogen production rate (SHPR) (42.69 mmol/g VSS.d), representing a 24%, and 43%, enhancement compared to control, respectively, was observed for the sole 50 mg Fe2O3/L supplementation. Furthermore, the co-addition of Fe2O3 plus NiO NP on biohydrogen production implies that the maximum HY and SHPR of about 17.2 mmol/g COD, and 47.67 mmol/g VSS.d, respectively, can be obtained from dairy wastewater at concentration of 50 mg Fe2O3/L, and 10 mg NiO/L, respectively. The highest significant relative enhancement of HY, 27%, and SHPR, 59%, suggests that the co-addition of hematite plus nickel oxide nanoparticles is about 1.2-1.5-fold more effective for enhanced hydrogen recovery from dairy wastewater compared to control, and sole nanoparticles addition. Furthermore, an enhanced biohydrogen production during BHP test can undoubtedly be attributed to an enhanced activity of the ferredoxin oxidoreductase, ferredoxin, and hydrogenase enzymes by surface and quantum size effects of NPs. Copyright (C) 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.