In general, a microemulsion consists of oil, water, surfactant(s) and possibly cosurfactant(s). In this paper, a novel surfactant-free microemulsion (SFME) is reported, which is composed of furaldehyde (oil phase), water and ethanol without a traditional surfactant of the amphiphilic molecular structure. The phase behavior of this ternary system was studied finding that there is a single-phase microemulsion region and a two-phase region in the ternary diagram. The electrical conductivity measurement was undertaken to investigate the microregion of the single-phase microemulsion region. On the basis of the percolation theory, the single-phase microemulsion region was subdivided into three different microregions: furaldehyde-in-water (O/W), bicontinuous region and water-in-furaldehyde (W/O), which were confirmed by freeze-fracture transmission electron microscopy (FF-TEM) observations. The sizes of the microemulsion droplets are in the range of 30-80 nm. Some surfactant-free emulsions (SFE) reported are O/W type and turbid, the significant apparent characteristic of SFME reported here is transparent, different from the SFE. The stability change of the SFME was not evidently observed after storage at room temperature for 22 months up to now.