A simple method for measuring the refraction indices of small hexagonal crystals is studied. The method is based on Brewster angle law. The refractive indices are confirmed by measuring the Brewster angle on two different surfaces. As an example, the measured refractive indices for hexagonal zeolite crystals are ne=1.488 and no=1.327, respectively, which are necessary for further investigations on the nonlinear properties of these zeolite crystal, such as second harmonic generation (SHG) effect. This method avoids the damage of the sample, and is simple and quick, which is suitable for the measurements of small birefringent crystals.