In this work, two Bronsted acidic ionic liquid (IL) catalysts containing benzimidazolium and imidazolium cation and two basic IL catalysts, i.e. 3-(2,3-dihydroxypropyl)1-methyl-1H-imidazole-3-ium hydroxide (DFImIL-OH-) and poly(3-benzyl-1-(4-(sec-butyl)benzyl)-1H-benzimidazole-3-iumhydroxide) (PIL(OH-), have been prepared. Developed catalyst was then characterized by NMR and FT-IR spectroscopy, and the acidity of BBAIL and ImBAIL was measured by the Hammett method using 4-nitroaniline as an indicator. Among all synthesized catalysts, benzimidazolium based Bronsted acid ionic liquid catalyst (BBAIL) was found to be best for the transesterification of castor oil in terms of biodiesel yield. Biodiesel yield was found to have affected significantly by castor oil to methanol molar ratio, reaction temperature and catalyst amount. Maximum yield of the biodiesel (i.e. 96%) was obtained using 1:12M ratio of castor oil to methanol at 40 degrees C and 5 mol% of the catalyst loading in 14 h. Obtained results indicated that BBAIL could be an efficient catalyst for the synthesis of much-desired biodiesel synthesis. Major highlights of this work are moderate reaction temperature, easy catalyst separation and catalyst recyclability up to 3 cycles with little loss of yield.