In this study, visible spectra are utilized to investigate the gel formation in the California Mastitis Test (CMT) for dairy cows. The milk and CMT solution will gel in proportion to the number of somatic cell count (SCC) in the milk. Most somatic cells are leukocytes (white blood cells), and they can reveal the severity of udder inflammation. Therefore, CMT is a simple cow-side indicator of SCC in milk. The test does, however, have a few limitations, such as including the subjective assessment of the strength of the gelling reaction between the gelling of the milk sample and the reagent, and it requires some time, slowing down parlor throughput. Also, the relationship between the subjective rating and the SCC (degree of udder infection) is not sufficiently accurate. Therefore, in this study, we objectively identify the relationship between the CMT gelling reaction and SCC using the nondestructive optical inspection technique to achieve a rapid and efficient online determination of the degree of udder infection. The investigation of parameters involves the absorption and transmittance of light that passes through the CMT solution with various SCCs. The intensity decreases systematically as the SCC increases, and it effectively predicts the SCC around 517 nm, but poorly predicts lower SCCs (<10 × 104 cells/ml) around 609 nm. This optical system using the visible spectrum may be utilized as an alternatively potential method for evaluating the CMT score.