With the rapid development of nanotechnology, nano-particles have increasingly been used in various fields. Zinc oxide (ZnO) nano-particles are the most widely used metal oxide nano-particles that enter the soil through various pathways, affecting the microbial activity and processes of the soil, such as carbon (C) mineralization. The C mineralization of organic residues by soil microorganisms is an important process in the carbon cycle, and any change in this process can affect the stability of the ecosystem. The purpose of this study was to investigate the effects of ZnO nano-particles on CO 2 released from microbial respiration (C mineralization rate) and soil microbial biomass carbon (MBC). For this purpose, soil samples were treated with 10 g kg -1 of wheat and alfalfa plant residues. Zinc oxide nano-particles were added to the samples with concentrations of 0, 100, 200, 400, 600, 800 and 1000 mg kg -1 soil. Soil moisture content adjusted at 60% water holding capacity (WHC) and incubated for 25 days at 25 ° C. During this period, carbon dioxide from microbial respiration was measured at 1, 2, 3, 4, 5, 6, 7, 8, 11, 14, 17, 22, 29, 36, 43, 58 days. Microbial biomass carbon of the soil samples was measured over a period of 10 days at the end of the incubation period. The effective concentrations to cause inhibitions by 50% (EC 50 ) ??and carbon mineralization kinetic parameters were calculated from the concentration-response and the first-order kinetics models, respectively. In addition, soil metabolic quotient (qCO 2 ) was calculated by dividing soil microbial respiration by microbial biomass carbon. The results showed that soil microbial respiration of the soils containing alfalfa residues was more than those containing wheat residues. Microbial biomass carbon in the soil containing wheat residues was higher than the soils treated with the alfalfa residues. With increasing the concentration of ZnO nano-particles, soil MBC value decreased while the metabolic quotient increased, suggesting that the nano-particles had toxic effects on soil microorganisms. Zinc oxide nanoparticles also affected the soil carbon mineralization kinetic parameters and reduced the potentially mineralizable C (C 0 ) while increased the rate constant (k) of C mineralization. The EC 50 values? for the effects of ZnO nano-particles on soil microbial biomass containing wheat and alfalfa residues were calculated as 178 and 1166 mg ZnO kg -1 soil, respectively. The EC 50 was calculated as 200 mg ZnO kg -1 for microbial respiration of the soil containing alfalfa residues. In general, the results of this study showed that the effect of ZnO nano-particles on the mineralization of organic carbon in the soil depends on the type of plant residue and the concentration of nano-particles entering the soil. Key Words : Nanoparticles, Zinc Oxide, Carbon Mineralization, ED 50 , Kinetic parameters