Composting is one of the oldest methods of recycling of plant residues and organic wastes.Compost is an environmentally safe and stable soil amendment produced from microbial decomposition of different degradable organic residues with a brown color and a soil odor. Compost is one of the most stable organic sources which is in long term increases the soil organic carbon. In addition to containing the essential nutrients for plant growth and developments, this organic amendment has a high water capacity that increases the available water for plants in soil. In this research the effects of particle size and types of plant residues on chemical and biological properties of compost in two production methods were studied. The experimental composts were produced using a conventional box method as well as a laboratory scale reactor. The conventional box method is an aerobic compost production method applying manual aeration, while reactor is a relatively new and more expensive tool that manages the compost temperature with self heating by the means of heat insulation, and also the compost aeration using an automatic electrical air compressor. In this work the effects of three types of plant residues as: pine, mulberry and elm with two particle sizes as: smaller than 1cm and 1-2cm, mixed with relatively fresh cow manure, on the produced compost quality were studied. The study was carried out in a Factorial completely randomized design with repeated observations over time with12treatments and threereplications. The treatments were threeplant residues aspine, mulberry and elm, two groups of particle size as: smaller than 1cm and 1-2cm and the compost production methods as: reactor and conventional box. The above mentioned treatments were prepared and incubated for 90 days for compost production. During the incubation period the reactors were automatically aerated for 15 minutes a day using an electrical air compressor. Also sampling, mixing and wetting of different treatments were carried out every 10 days, manually. The samples were then dried at 60-70ْC in an electrical oven, powdered and analyzed for some chemical and biological properties as: pH, EC, organic carbon, nitrogen, carbon to nitrogen ratio, phosphorus, potassium, iron, zinc and lignin contents. Seed germination test was also carried out in final composts of different treatments. Statistical analysis performed using SAS and Mstatcsoftwares and comparison of means using the least significant difference (LSD) at 5% probability. The results indicated that composts produced in reactor generally showed better chemical and biological properties than the conventional box composts. The pine residues showed a better decomposition than the other residues during the incubation period. All composts turned to a brownish color with a soil odor showing a relatively good maturity at the end of incubation period. The rate of seed germination was higher for composts produced in reactor as compared with the composts produced in the boxes. It was also higher in pine residues composts than the other two residues composts. The particle size did not induce any significant difference in composts chemical properties except for potassiumand phosphorus contents. The pH, EC, carbon, nitrogen, phosphorus and potassium contents and carbon to nitrogen ratio of the produced composts matched the related international and Iran standards, so the produced composts could be Keywords Composting methods, Seed germination test, Organic fertilizers, Soil organic amendment, Agricultural wastes