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Antibacterial resistance is widely emerged in the Acinetobacter baumannii.Clinicans are resorting to use older agents such as colistin, or employing unorthodox combinations of licensed durgs to bridge the current treatment gap.At lower concentrations, colistin effects on the bacterial outer membran and permeates Gram-negatieffects on the bacteria, by which facilatetes the entering of the compounds which usually excluded.Our present study explore the combination effects of colistin with vancomycin, norvancomycin or linezolid against 40 clinical isolated multidrug-resistant Acinetobacter baumannii (MDR A.baumannii) by using fractional inhibitory concentration index (FICI) and time-kill assay.The results indicated that minimum inhibitory concentrations (MIC) of vancomycin and novancomycin for half of isolates fell down below the susceptibility break point.MIC of linezolid for one isolate fell down to blood and epithelial lining fluid concentration by the current dosing regimen.When vancomycin or norvancomycin was used in combination with subinhibition doses of colistin, the combinations could eradicate the test MDRA.baumannii.Transmission electron microscopy studies showed that.subinhibition doses of colistin were able to disrupt outer membrane, which facilitates vancomycin disrupting the cell wall leading to cell lysis.In conclusion, subinhibition doses of colistin significantly enhance the antibacterial activity of vancomacin, norvancomacin, linezolid against MDR A.baumannii.