Polymorphism of kappa casein protein (kappa-casein) in total 113 Brown-Swiss cattle was investigated using horizontal starch-urea gel electrophoresis method to verify its effect on milk production traits (actual milk yield, 305 d milk yield, actual fat yield, 305 d fat yield, fat percentage and lactation length) and influence of other factors such as parity and season of calving; aiming at utilizing it as a genetic aid in selection to improve the milk production traits of this breed. The allelice frequencies of A, B, and C were found to be 0.345, 0.633, and 0.022 respectively. Genotype frequencies were in accordance with the Hardy-Weinberg equilibrium. General linear model (GLM) was used to analyze differences between genotypes. The results indicated that kappa-casein genotypes significantly (P<0.05) affected 305 d milk yield, actual fat yield, 305 d fat yield, fat percentage, and lactation length. But it had no effect on actual milk yield. Parity and season of calving had effect (P<0.05) on milk production traits. This study indicated that the kappa-casein genetic variants may be used as a genetic aid through increasing the frequency of desired genotypes to improve the milk production traits of this breed.