Composting of two-phase olive mill pomace, poultry and dairy manure, and straw was conducted at five initial carbon/nitrogen (C/N) ratios (from 20.03 to 39.99) using 15 100-liter reactors. Compost temperature was controlled based on Rutgers aeration strategies. Composting trials lasted 19.35 days. A first-order rate equation was used to obtain the decomposition rate and compost mass ratio (the degree of advancement of the composting process). The compost stability index was calculated considering the decomposition rate and the rate of oxygen uptake per unit of dry matter loss based on biochemical oxygen demand. The nonlinear relationship between the compost stability index and the initial C/N ratio showed that a minimum value of 0.011 kg O2 day−1 kgc−1 existed at the initial C/N ratio of 28.92. The lower compost stability index indicated relatively more stable compost. The relationship between the final (Formula presented.) ratio and the initial C/N ratios indicated that the lowest final (Formula presented.) ratio of 0.26 occurred at the initial C/N ratio of 29.66. The germination index as a function of the initial C/N ratio yielded a strong negative linear relationship with R 2 of 0.95. Compost mixes with the high initial C/N ratio resulted in relatively lower germination indexes which are phytotoxic to plant growth. This study showed how the initial C/N ratio is important for compost stability and maturity.