A method for measuring the minimum deviation angle and making the precise determination of refractive index of optical material from ultra-violet to infra-red is given. The region of wavelength are from 0.21 to 2.43μm. The dispersion formula of fused silica and CaF2 crystal have been evaluated. The measuring results coincide with the calculation very well. The measurements have been done by an instrument consisting of a reflective collimator and a GM-1 spectroscope. The accuracy is ±2×10 -5 when λ<2 μm and is a little lower when λ>2 μm.