Ketosis in dairy cows can lead to poor reproductive success and decreased milk production. Since the serum concentrations of copper (Cu) and zinc (Zn) are closely associated with the health status of cows, we investigated whether serum concentrations of Cu and Zn differed in dairy cows with subclinical ketosis and healthy dairy cows. Blood samples of 19 healthy dairy cows and 15 subclinically ketotic dairy cows were collected from three farms, and the concentrations of β-hydroxybutyrate (BHBA), glucose, non-esterified fatty acids (NEFA), Cu, and Zn were determined. Subclinically ketotic dairy cows had significantly higher BHBA and NEFA levels (p < 0.01) and lower glucose (p < 0.01) than healthy dairy cows. Likewise, serum concentrations of Zn were significantly decreased (p < 0.05) in dairy cows with subclinical ketosis. There was no significant difference observed for serum Cu concentration between healthy and subclinically ketotic dairy cows. This study suggests that a decreased serum Zn concentration could be a cause of decreased reproductive performance in subclinically ketotic dairy cows.