Three sulfated titania-based solid superacid catalysts have been prepared by sol-gel and impregnation method. The crystallization of the catalysts has been analyzed by x-ray powder diffraction, and acid strength by temperature-programmed desorption of ammonia and Fourier transform infrared spectroscopy, respectively. The catalytic performances for the synthesis of biodiesel from rap oil have been evaluated at 353 K under atmospheric pressure in a three-necked round-bottomed flask using a 1:12 molar ratio of oil to methanol. The results show that the immersed sulfated titanium exhibits the highest activities among these catalysts. The characterization of temperature-programmed desorption of ammonia and Fourier transform infrared spectroscopy reveals that the strong acid site responds to catalyst reactivity.