The effects of three kinds of cosurfactants (n-butanol, isobutanol and sec-butanol) and their additions on the phase behavior of microemulsion were explored. In the microemulsion, fatty alcohol polyoxyethylene ether (AEO-9) was used as surfactant, NaCl as inorganic salt, octane and diesel as oil phase. It was found that with the increase of the addition of cosurfactants, the phase behavior of the microemulsion system changed from Winsor I through Winsor III to Winsor II. In octane microemulsion systems, the maximum volume fractions of the microemulsion phase were 91.63%, 80%, and 65.49% corresponding to n-butanol, isobutanol and sec-butanol respectively. Similarly, in diesel microemulsion systems, the maximum volume fractions of the microemulsion phase were 68.42%, 68.29%, and 63.33%, respectively, which suggesting that these three kinds of cosurfactants microemulsion systems have better solubilization properties for octane. The formulations of Winsor I and Winsor III microemulsion were screened out for each cosurfactant and oil phase by alcohol scanning experiments, and the effect of oil/water ratio on the phase behavior was explored. It was found that with the increase of the oil/water ratio, the final phase type of microemulsion was Winsor I. The microemulsion system which was initial Winsor Ш needed lager oil/water ratio to change into Winsor I microemulsion.