Microbial biomass carbon (MBC) and microbial quotient has been utilized as sensitive indicators to changes in ecosystems that are considered in soil quality studies. This study aimed to investigate the effects of selection system on the seasonal and monthly variations of MBC and microbial quotient in mixed beech stand which was performed in 2013. For this purpose, two stands including control and forest managed by selection system in Dr. Bahramnia educational and research forest in Gorgan province, Iran. We selected 6 (3×3 meters) sample plots within each stand (3 sample plots under closed canopy and 3 sample plots under the canopy gap with an area about 400 square meters. Three soil samples were taken from 0-10 cm depth at three random points within sample plots. For each soil sample, MBC, organic carbon, soil temperature and moisture were measured monthly in each sample plots within one year. Results showed that there were many fluctuations in MBC and microbial quotient in different seasons and months through the year despite the inconsiderable variation of soil organic carbon (3.01-4.55 percent in control stand and 3.04-4.51 percent in the managed stand). The highest and least amount of these two parameters were in autumn and winter, respectively. MBC mean in control stands (933.56± 22.66 mg.kg^-1) were statistically similar with managed stand (940.44± 22.92 mg.kg^-1) through the year. The results indicate the potential of managed stand in maintaining the balance after implementing the selection system method and the success of the implementation of this method based on these biological indicators.