The possibility was examined of using calcium oxide (CaO) as an environmentally friendly catalyst to produce biodiesel fuel (BDF) from oils and fats. The calcium oxide, CaO-E, produced industrially by calcination of limestone in an ambient air, catalyzed the transmethylation of rice oil, but the yield of fatty acid methyl ester (FAME) was only 8.3% in 2 h. However, enhancements of the base amount (>0.77 mmol g(-1) of the base strength 7.2 <= pK(BII)<= 15.0), specific surface area (>20m(2)/g), and total pore volume of 2-200 nm pore size (>0.19 mL/g) of CaO were achieved by the calcination of calcium hydroxide (Ca(OH)(2)) either in vacuo at 425 degrees C for 9 h or 500 degrees C for 1.5 h, or in an atmosphere of nitrogen at normal pressure and 600 degrees C for 4 h correspondingly to the Ca(OH)(2) industrially produced by hydration of the CaO or reagent-grade Ca(OH)(2) (CaO-A, CaO-B, CaO-C, and CaO-D). These calcium oxides exhibited very high catalytic activities in rice oil transmethylation with methanol; the conversion of FAME reached >96% in 2 h.