A careful analysis demonstrated that when optical beams propagated with the wave vector parallel to the some orientations in biaxial crystals, they would not exhibit the first-order thermal-induced static phase retardation. These orientations were now called NTSPR directions of the crystals. In this paper, taking some approximations, we theoretically founded the NTSPR directions in KNbO3 and KTP crystals, with the help of the relative data from the literature. Also, the EO figure of merits at these NTSPR directions in KNbO3, KTP crystals is discussed. The calculations implied that with the large EO figure of merits at these NTSPR directions in some biaxial crystals, the novel single-crystal EO switches would have short propagation length and need not to be temperature-controlled compared to EO switches of traditional structure.