Abstract: Abstract: The aim of this study was to find the optimal conditions for cotton straw composting as a Soilless Culture Substrate. Cotton straw size, carbon-nitrogen ratio, and nitrogen source were investigated to determine their effects on the process of cotton straw composting by using an orthogonal design method. Each factor was set at three levels: C/N ratios were 25:1, 30:1, 35:1, cotton straw sizes were 1, 2, and 3 cm, and nitrogen sources were chicken manure、urea, and a mixture of chicken manure and urea. Cotton straw applied in this trial was bought from the famers in a Beijing suburb and was broken into 1-3cm particles by machine. The C/N ratio of cotton straw was 38:1. The dry chicken manure and urea as the nitrogen resource were used to adjust the C/N ratio. The cotton straw weight of each treatment was 5 kg, and the water content of each treatment was adjusted to 60%-70%. Plastic weaving bags of 70-liter capacity were used as composting containers and were placed in three layers with a randomized design. Each treatment had one bag and three replications. The bags were turned over every 10 days during the maturation phase in order to improve the O2 level inside the bags. The trial lasted 30days.The parameters included composting temperature, C/N ratio, bulk density, pH, EC, accumulated temperature, water holding capacity, and air filled porosity. A temperature meter recorded the temperature in each bag every day. Bulk density and porosity were determined following the Byrne method and conventional method, respectively. The pH and electrical conductivity (EC) were determined by IQ150 Portable pH/mV/thermometer measurement.The results from the study indicated that during the composting period, C/N ratio of 25:1, 1 cm straw particle size, the mixture of chicken manure and urea as an added nitrogen source were the optimal conditions to sustain high temperature (＞50℃) in the composting pile of cotton straw, and the days keeping high temperature in the three conditons was 9, 10, 8 days respectively. The C/N ratio had a significant influence on total porosity reduction, with the highest value (66.0%) in a C/N of 25:1, and that in pre-composting decreased by 24.8% compared with after composting. Cotton straw size had a significant influence on water-holding capacity、accumulated temperature and bulk density increase, and the highest values (59.2%, 1 315.75℃, 0.30 g/cm3 ) occurred with a cotton straw size of 1cm. Compared with pre-composting, the water holding capacity after composting increased by 111.1%, and the bulk density increased by 76.4%. Nitrogen source (chicken manure and urea) had a significant influence on accumulated temperature, and the highest measured value (1 354.41℃) occurred with the mixture of chicken manure and urea as the nitrogen source. Recommended values are a C/N ratio of 25:1, 1cm cotton straw particle size, and a mixture of chicken manure and urea as an added nitrogen source for cotton straw composting.