In the paper are presented the results of research on determining the role of effective microorganisms (EM) in the bioconversion process of biodegradable unfermented organic wastes (manure of cattle). For this purpose an experiment has been performed during one month, in which two concentrated solution of microorganisms („Baikal ЭМ-1” and „EM-1”) were used from which for preparing the basic and working solutions. The activity of the preparation „Baikal ЭМ -1”, was carried out under the anaerobic conditions, and of the preparation „EM-1” – in aerobic conditions. At the start of the experiment, in order to determine the quality of the manure used for processing with EM, it was subjected to biochemical analysis determining the following indicators: humidity, dry matter, active acidity, ammonia, total nitrogen, ash and organic substance. Analyzing the obtained results, we noticed that that in samples of unfermented manure the value of humidity, dry substance, active acidity, ammonia, total nitrogen, ash and organic substance constituted respectively 85.48%, 14.52%; 7.13 conventional units, 313.9%, 0.8 mg / kg, 13.52% and 43.24% After 7 days from the start of the experiment in the experimental variants was detected the elimination of important amount of gas, which reached the maximal rate at the 14th day. After 30 days, the activity of EM reduced, indicating the endpoint of the process. The obtained compost was subjected to biochemical investigations. Biochemical indicators of obtained compost in experimental variants changed, both in comparison with the initial phase and with the control variant. In the control manures, after a month of open fermentation, the value of dry substance, active acidity, total nitrogen and ash have increased respectively with 28.10% – 25.91%; 28.75% – 27.76%; 177.50% – 65.19% and 69.38% – 19.04% and the content of ammonia decreased with 86.06% – 87.60% comparatively with the values of indicators obtained in samples of manure at the initial stage of the experiment. In comparison with the control variant in samples of unfermented manure subjected to processing using preparations „Baikal ЭМ-1” and „EM-1”, the content of total humidity, organic substance increased, respectively with 4.67% – 3.26 and 12, 58% -11.56%, and that of dry substance, active acidity, ammonia, total nitrogen and ash decreased respectively with 20.43% – 25.90%; 33.01% – 31.35%; 79.76% – 83.20%; 37.84% – 39.01% and 42.36% -22.38%. Therefore, the analysis of the obtained results demonstrated that the activity of effective microorganisms from preparations „Baikal ЭМ -1” and „EM-1”, are manifested more intensively in the first period of the process of bioconversion of the biodegradable organic wastes and influenced beneficially the quality indicators of obtained compost.