By relying on dispersion equation, optimal conditions for negative refraction in the interface between isotropic medium and anisotropic crystal are investigated. The negative refraction phenomenon can be very evident when the optical axial angle and anisotropic parameters are arranged appropriately. The optimal optical axis angle and corresponding maximum incident critical angle are given. For strong anisotropy crystal, the maximum incident angle that yields the negative refraction could be very large. Further, the difference of negative refraction associated with uniaxial crystal and with negative index media is discussed. In negative index media the negative refraction is caused by a negative refractive index, whereas in the case of anisotropic crystal, the negative refraction is a natural result of dispersion relation in anisotropic media. It is found that the uniaxial crystal slab is not suitable for constructing perfect lens, firstly predicted by Pendry.