This study presents the formation and phase behavior of vegetable oil-based ionic liquid microemulsions (ILMs). Castor oil, jatropha oil, and soybean oil were used as oil phases of the ILMs respectively. The effects of the mass ratio of surfactant to cosurfactant (K-m) and temperature on the phase behavior of ILMs were investigated by pseudo ternary phase diagrams. The results indicated that the homogeneous and stable microemulsions composed of target vegetable oils, ionic liquid, Triton X-100, and n-butanol could form at ambient condition successfully. The size of a single-phase region area was in a sequence: castor oil-based ILMs > jatropha oil-based ILMs > soybean oil-based ILMs. Each vegetable oil-based ILMs corresponded to an optimum K-m since the ionic liquid-oil amphiphilic balance existed in ILM systems. A maximum single phase region area of each system was observed when K-m = 1:0, 2:1, and 2:1, respectively. Moreover, a larger single phase region area of vegetable oil-based ILMs could be obtained by increasing the temperature.