In laser cavity, the birefringent plate rotating about its surfacr normal can tune the oscillation wavelength. The influence of the orientation of optic axis with repect to the plate surface on the tuning characteristics is analysed in the present paper. It is shown that there are two optimum orientations of optic axis according to ray theory. The tuning characteristics of the plate with one of the optimum orientations of optic axis are better than those with other orientations. A design example of three-plate-filter and its tuning characteristics for Ti:Al2O3 laser are given.