The microbial influence of stability in metalworking fluid (MWF) was studied by adding the waste metalworking fluid into the MWF without biocides,and the microbial diversity in cutting fluid and the degradation of cutting fluid by microorganism was analyzed.Results show that dominant bacteria are identified as Pseudomonas sp.,Comamonas sp.,Rhodococcus erythropolis.Severe biodegradation are found in the MWF during 12 weeks of enrichment.Total organic carbon (TOC),pH of the MWF present a declining tendency under the microbial influence while inorganic carbon (IC) exhibits an increasing tendency.The microbially influenced destabilization of emulsion occurs during the first two weeks.Turbidity is increased dramatically and medium and maximum of droplet size distribution are 7 and 20 folds greater than in fresh MWF.The coalescence in droplets and free state oil is found in the MWF containing microorganisms.The mechanism of microbial destabilization in the MWF was discussed by the DLVO theory and hydrodynamic model.It can be informed that dielectric constant is increased by microbial degradation of organic substance,in which endows more activation energy to emulsion particles.The coalescence of particles occurs when the activation energy of particles crosses over barrier potential.