This study investigates the effects of the structures of different surfactants on the stability of the naphthen ic oil-based microemulsion metal cutting fluid system, which consists of water, oil, and an emulsifier.Sev eral kinds of common nonionic and anionic surfactants with different head and tail structures were used to e mulsify the naphthenic base oil.All combinations of these surfactants and oil were formulated using the full range of the oil-to-surfactant ratios and surfactant concentrations.The stability of each formulation was e valuated based on visual transparency and droplet diameter.The following set of surfactant-selection guide lines, which, if followed, would yield stable metalworking fluid (MWF) microemulsions with minimum re quired surfactant concentrations, was formulated based on the experimental results : (1) a single nonionic sur factant should be used to emulsify the naphthenic base oil, and the emulsifying effect follows the trend,straight-chain alcohol ether > branched-chain alcohol ether > aromatic ether ; (2) a combination of two surfactants is preferred over a single surfactant because a binary system reduces the amount of surfactants and enhances the emulsifying effect; (3) the anionic surfactant is used as a cosurfactant in the combination be cause the emulsifying effect depends on the water solubility and critical micelle concentration of the anionic surfactant, i.e., the higher the water solubility and the lower the critical micelle concentration the greater the emulsifying effect; (4) linear, isomeric fatty alcohol polyoxyethylene ether, alkyl phenol polyoxyethylene e ther, and fatty acid polyoxyethylene ester, which have the same ethylene oxide (EO; hydrophilic group -CH2CH2O-) numbers and different carbon chain, should be combined with an anionic surfactant to emul sify the naphthenic base oil.The emulsifying effect depends on the length of the carbon chain and on the tail structure of the surfactant.The higher the degree of similarity between the tail structure and the oil, the grea ter the emulsion; (5) linear, isomeric fatty alcohol polyoxyethylene ether, alkyl phenol polyoxyethylene e ther, and fatty acid polyoxyethylene ester, which have the same carbon chains but different EO number should be combined with an anionic surfactant to emulsify the naphthenic base oil.Excessively large (EO > 20) or small (EO < 5) EO groups do not yield stable emulsions; and (6) The foam height and duration as well as the low surfactant-to-oil ratio of the alkyl phenol polyoxyethylene ether series should be considered in com bining naphthenic oil with an anionic surfactant to achieve the optimum efficiency because smaller foams that can easily dissipate are formed.These guidelines can be used to design MWF microemulsions and lubricant formulations.