White blood cell (WBC) count will greatly facilitate the timely diagnosis and management of health conditions of dairy cattle, including clinical mastitis. We propose a WBC counting device for dairy cattle using a microchannel with field-flow fractionation incorporating light scattering, which allows both the separation and counting of WBCs. The performance characteristics of the microchannel were optimized to separate particles by size through three-dimensional (3D) numerical simulations. The fabricated microchannel was evaluated using three mock beads with sizes of 3.5 μm representing milk fat globules (MFGs), 10 μm for WBCs, and 30 μm for cancer cells. The pinched flow microfluidic device was useful for separating particles with diameters of 10 and 30 μm. The WBC count was determined using the fabricated WBC counting device. Light scattering was effective for separating cells of 10 μm diameter or less such as MFGs and WBCs that pass through a close streamline. The linear regression analysis of the calibration curve for the WBC counting device showed a multiple determination coefficient of 0.98. The performance of our WBC counting device approached that of laboratory-based tests, allowing the quantitative and rapid analysis of the concentration of WBCs in raw bovine milk in field applications.