This study was conducted to determine the effects of C/N ratio and free air space in co-composting of sewage sludge with tomato stalk and dairy manure. Experiments were carried out in 100 L of stainless steel aerobic compost reactors with full automation system and monitored for 32 days. The temperature was controlled according to the Rutgers strategy. During the composting process, moisture content, organic matter content, pH, electrical conductivity, total carbon, total nitrogen, C/N ratio, total phosphorus, potassium, NH4+ - N, NO3- - N and heavy metals contents were determined. For evaluation of the stabilization process, organic matter, dry matter, ammonia and mass and volume losses and temperature index values were taken into consideration. The temperature pattern in the mixtures with dairy manure increased rapidly and reached higher levels depending on dairy manure ratio. The highest organic matter loss was 57.87%, which was in the mixture with a C/N ratio of 20 and a free air space ratio of 37%. (C) 2018 Elsevier Ltd. All rights reserved.