Negative refraction and imaging properties of uniaxial crystal slabs are investigated by the refractive index ellipsoid analyses. It is found that the concept of negative refraction can be extended to be an intrinsic property of all uinaxial crystals. The negative refraction effect of uinaxial crystals is mainly due to the anisotropic refractive index. The angular range for incident light to yield negative refraction attains its maximum which only depends on the difference of two refractive indices and the orientation of the crystals with their optic axes at a certain angle to the normal of the light incoming surface. The negative refraction imaging of uniaxial crystal slabs is restricted within some special conditions. These conclusions are verified by the experiments on positive uniaxial crystal YVO4 and negative unaxial crystal CaCO3.