Aims: We aimed to systematically understand the composting processes by a comparison of microbial communities during four full-scale composting processes. Methods and Results: Microbial communities during the four different full-scale composting processes were analysed by denaturing gradient gel electrophoresis combined with measurement of physicochemical parameters. Two composting processes utilized sewage sludge and two utilized food-waste. Comparison of the four processes indicated that the concentration of dissolved organic carbon was higher in the food-waste-composting than in the sewage-sludge-composting processes, and microbial communities varied with composting substrate. The tendency for different microbes to appear in the composting process with different concentrations of dissolved organic carbon agreed with a previous study that showed that microbial succession occurred with a decrease in dissolved organic carbon in a laboratory-scale food-waste-composting process. Conclusions: Our results suggested that the main factor affecting microbial communities in the composting process is the concentration of dissolved organic materials. Significance and Impact of the Study: In addition to studying microbial communities involved in composting, this research is also the first to study composting mechanisms using molecular methods. The results of our studies may be helpful in the design and management of composting processes.