Abstract: Four ethanol consuming microorganisms (GR13, GW13, GT17, and GY22) were isolated from the contaminated culture of genetically engineered Synechocystis sp. and were utilized to evaluate the effects of these microorganisms on the ethanol consuming capabilities and ethanol productions of Synechocystis sp. Based on the morphological characteristics and 16S rDNA and 26S rDNA gene sequence, the four strains were identified as Rhodotorula sp., Meyerozyma sp., Brevundimonas sp., and Microbacterium sp., respectively. We observed that strain GR13 (Rhodotorula sp.) had the highest level of specific ethanol consumption rate 391 g/(1015 cfud), followed by strain GW13 (Meyerozyma sp., 80.1 g/(1015 cfud). While strains GT17 (Brevundimonas sp.) and GY22 (Microbacterium sp.) had much lower ethanol consumption rate. All four strains did not significantly affect the growth of Synechocystis sp., however, they remarkably decreased the ethanol production of Synechocystis sp. The results of the co-culture experiment demonstrated that strains GR13, GW13, GT17, and GY22 diminished ethanol production of the genetically engineered Synechocystis sp. by 53.8%, 23.6%, 40.7% and 27.3%, respectively.