The paper presents a derivation of the locus equation of rectangular waveguide of which index distribution function is n(x) = no (1 - 1/2 Ax3). The locus of light rays can be described with logarithmic function as incidence plane is the central plane having refraction index no, and tne locus of light rays can be indicated with arc tangent function as incidence plane is in edge which has index of n(X). When tangential plane crosses no plane, the locus of paraxial rays locating in tangential plane is a well-known sinuous function. According to the locus equation^ this waveguide has a function similar to parallel plate or divergent lens or cylindrical lens. When it is used as parallel plate, the waveguide offers much larger displaceiueiiii than that given by a parallel plate wMch made of homogenous medium. As a divergent lens, the waveguide can not be used as image-forming element, becaug? it only acts when light rays have a certain angle with optical axis. The image-forming feature depends on the distribution constant A, the central refraction index n0 and the thickness Z0 Thus, the locus line may locate in the outside or inside or end of the medium. The waveguide may be used in optical systems. The paper places emphasis on deriving locus equation of light ray motion in the inside of tho medium according to eikonal equation.