Abstract: Mixtures of pig or cow manure and wheat straw, prepared in two different ratios, 75:25 and 50:50, were used in a composting experiment, with or without inoculation of efficient cellulose-lignin degrading bacteria. The experiment lasted for 28 days. Results show that with composting going on, the total organic carbon (TOC) displayed a declining trend in all the treatments, while humus carbon increased its proportion in TOC; Inoculation of the microbes promoted mineralization of TOC and formation of humus. The humic substances accumulated more in the mixtures with microbial inoculation than without inoculation, which shows that the microbial treatment tends to increase the proportion of humic substances. The ratio of humic acid to fulvic acid (H/F) increased with the decomposition process in the mixture of cow manure and wheat straw, while it increased first and then decreased in the mixture of pig manure and wheat straw. At the end of the composting, the treatments with inoculation were higher in H/F than those without inoculation, indicating that microbial inoculation promoted humic acid formation. To sum up, a high proportion (75:25)of manure in mixture plus microbial inoculation can promote formation of humic substances, which is more obvious with the mixture of cow manure and wheat straw than with the mixture of pig manure and wheat straw.