In this study, the rich carbon content of rice straw and the high nitrogen content of sewage sludge make the straw a good potential substrate and the sludge a viable inoculum for biohydrogen production. Two treatment conditions for the sewage sludge (raw and heat-treated) were used in the present experiments. Batch test using a mixture of rice straw and sewage sludge were carried out to investigate the optimum carbon to nitrogen (C/N) ratio for effective biohydrogen production. The experimental results indicate that untreated sludge could be used as the inoculum for efficient hydrogen production when mixed with the appropriate proportion of rice straw. According to our results, biogas and hydrogen production in all raw sludge cases ramped up more quickly and also exhibited longer and higher hydrogen production in comparison with heat-treated cases. At the C/N ratio of 25 in untreated sludge, hydrogen production was 33% higher than heat-treated one. Additionally, under the same conditions, high and stable hydrogen content (58%) and the maximal hydrogen yield (0.74 mmol H-2/g-VS added straw) were obtained. Copyright (C) 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.