An investigation on a microemulsion system consisting of sodium dodecyl sulfate (SDS), butyl lactate (BL), water, and isopropyl myristate (IPM) was performed to evaluate butyl lactate as a cosurfactant. The pseudo-ternary phase diagrams revealed that the so-called U-type microemulsions consisting of single isotropic region could be obtained. In addition, an increased weight ratio of BL to SDS induced wider regions of microemulsion. For characterizing the W/O microemulsions with BL, electrical conductivity and dynamic light scattering (DLS) measurements were performed. In the DLS work, a new method was proposed for accurate evaluation of the inherent viscosity of continuous oil phase by utilizing the ratio r = [cosurfactant]/[SDS] at the interface of W/O microemulsions from the conductivity study and the viscosity data for the blended solution of BL and IPM. The conductivity study Shows that the ratio r was near 2 and decreased to 0.4 upon increasing the weight ratio of (SDS+BL) to IPM. The size of the W/O microemulsions was relatively small, ca. 4~16 nm, and the size was a function of the volume fractions of water and the total concentrations of SDS and BL. The present work shows that a wide microemulsion region can be attained when using BL as an cosurfactant. These Microemusion systems, owing to the skin compatibility of BL, should be very effective when they are employed for dermal drug delivery and cosmetic applications.