The aim of this study was to determine the optimum mixture ratios for composting separated cattle manure and pepper plant residues. Five 127 liter laboratory-scale bioreactor were used to investigate of separated dairy manure addition on the composting process of greenhouse pepper plant residues. Cattle manure is considerably rich in nitrogen. However, composting with cattle manure alone is not suitable due to its low C/N ratio and FAS (free air space) value. For this reason, greenhouse pepper plant residues was added as a source of carbon in order to balance the C/N ratio and FAS value of the mixture. During the composting process, the process temperatures, CO2 concentrations moisture and organic matter content of the mixtures were monitored. Results showed that a mixture consisting of 60% cattle manure and 40% greenhouse pepper plant residues allowed for the highest process temperature and organic material decomposition. Consequently, this mixture was determined as the optimum mixture for the composting process.