Castor oil-based ionic liquid microemulsions are promising alternatives for petroleum-based biolubricant basestocks. This study presents the phase behavior Of castor oil-based ionic liquid microemulsions through phase manifestation, and the areas of the single-phase domain (S-ME) were calculated accordingly to further illustrate the phase-forming capacities of the designed microemulsions. The results indicated that the phase-forming capacities of castor oil-based ionic liquid microemulsions depended largely on the ionic liquid ions, surfactant types) and cosurfactant chain lengths. The S-ME of different anion-based ionic liquid microemulsions showed the following sequences: [BMIM][Tf2N]-based > [BMIM][PF6]-based > [BMIM][BF4]-based. The longer carbon chain of ionic liquid cations in single surfactant-based systems and the larger percentage of ionic liquid-based surfactant in mixed surfactants-based systems both gave rise to the phase-forming capacities of castor oil-based ionic liquid microemulsions. Given the presence of ionic liquid-castor oil amphiphilic balance in the designed systems, the castor oil-surfactant micelles achieved maximum solubilization capacity for [BMIM] [BF4] when the ethoxylated groups' number of surfactant was about eight in single surfactant microemulsion. In addition, n-hexanol donated a higher phase-forming capacity than n-butanol, while n-octanol brought about a different phase behavior due to the spontaneous curvature effect and oil nature of long-chain alcohols (C >= 7) in the castor-oil based ionic livid microemulsions. Thus, this study presented useful information for formulating optimum castor oil-based ionic liquid microemulsion biolubricants based on different ionic liquids, surfactants, and cosurfactants, which were not evaluated in previous research.