AbstractGraded refractive index rod (GRIN‐rod) lenses are used widely in optical devices for optical fiber communication systems. Insertion loss is a major criterion for evaluating optical devices, especially those used in single‐mode fiber systems, and aberration is a dominant factor in insertion loss. Thus, aberration analysis is essential in designing high‐perfomance optical devices. This paper presents ray traces within the third‐order aberration when light propagates through GRIN‐rod lenses. Using the fact that the ray is the orthogonal trajectory of the wavefront, we derive a novel wave optical description of GRIN‐rod lenses to the degree of third‐order aberration. This paper shows the relatinship between coupling efficiency and lens parameters. The necessity of considering aberration effects in designing low insertion loss optical devices is shown. The experimental results are in very good agreement with the calculated values, and the validity of the present analysis is confirmed. This paper proves that GRIN‐rod lenses are superior to ball lenses in terms of aberration, and provide higher coup