This research was performed to estimate the shape of an average lactation curve and to develop adjustment factors to reduce errors in estimating test-interval milk yields from sample-day production. Complete records were available for 187 lactations for the period from calving to 310 days in milk. The average daily production increased rapidly from calving to a peak at 39 days in milk and declined at a nearly linear rate. To estimate test-interval yields accurately adjustment for the location of sample days on the lactation curve, especially the interval from talking to first sample days, interval around the lactation peak and interval after last sample day were required. Separate factors were developed for the three interval,; of the lactation curve that require the adjustment. Factors from this study were similar to USDA factors but different from those adopted in Korea Animal Improvement Association.