A total of 2,399 records from 337 Holstein cows were used to estimate variations of milk components during test period. The cows were tested at National Livestock Research Institute(NLRI) from 1996 to 1997. The milk components included fat(3.7±0.95%), protein(3.2±0.2%), lactose(4.7±0.0%), Solid-Not-Fat(8.7±0.1%) and total solids(12.0±0.8%), and their coefficients of variation were 26.1%, 9.1%, 1.6%. 1.9%, 7.3%, respectively. The fat percentage of milk components varied very high, but lactose content varied much less. The components of fat, protein and total solids were high in early and late stage the during test period, but that of lactose was high in the mid stage.