To investigate the influence of different monocarboxylic acids on the single-phase microemulsion region of a 1-butyl-3-methylimidazolium hexafluorophosphate(bmimPF6)/Tween80/toluene system, pseudo ternary phase diagrams consisting of bmimPF6, toluene and Tween80 at 25℃ in the presence of monocarboxylic acids were constructed, monocarboxylic acids used were CH3COOH, n-C3H7COOH, n-C5H11COOH,or n-C7H15COOH. Effects of monocarboxylic acid chain length and contents of n-C3H7COOH on the formation of single-phase region were investigated. Phase diagrams demonstrated the single-phase domain became smaller as the n-C3H7COOH /Tween80 weight ratio increased. Different chain length of monocarboxylic acids affected the area of single-phase regions, which was dependent on partitioning in biphasic ionic liquid and oil system and lipophilic property of the monocarboxylic acid. The single-phase area decreased as the monocarboxylic acid chain length increased. The single-phase region of the n-C3H7COOH-containing system was identified by electrical conductivity. There were oil-in-ionic liqid microemulsions(O/IL),bicontinuous(BC), and ionic liquid-in-oil (IL/O) microemulsions in the single-phase regions. In the IL/O microemulsions.Dynamic light scattering showed the droplets initially formed were linearly enlarged by the addition of bmimPF6. However, large-sized droplets with deviation from the linearity , appeared with successive addition of bmimPF6