The degradation of the paper fraction was investigated in the composting of source separated food waste and paper mixture with change of C / N ratios. In composts with low C / N ratio of 14.0, the cellulose in the paper fraction was degraded in the maturing stage concurrent with peak cellulase activity, temperature rise, and sub-peak of CO2 evolution rate. This result might be ascribed to the initial low pH and high ammonia content caused by the abundance of readily degradable organic matter. However, the cellulose was degraded mainly at the thermophilic stage in composts with C / N ratios of 20.4 and 29.9, indicating that these ranges of C / N ratio might also be appropriate for cellulose degradation. The cellulase activities increased abruptly during the thermophilic stage and reached maximum value al the cooling stage in composting materials of appropriate C / N ratios. Cellulose was degraded significantly during the increase of cellulase activity.