The objective of this study was to estimates the genetic and environmental components for the LACTATION CURVE PARAMETERS, milk yield, age at first calving (AFC), calving interval (CI) and somatic cell count (SCC) in Iranian Holstein cows. The dataset consisted of 210625 test day records from 25883 cows with milk yield in the first parity recorded from July 2002 to September 2007 in a total of 97 herds in Iran. The LACTATION CURVE and the selected LACTATION PARAMETERS were the scaling factor to represent yield at the beginning of LACTATION (a), the factor associated with the inclining (b) and declining (c) slopes of the LACTATION CURVEs and the first 100-day milk yield, second 100-day milk yield, third 100-day milk yield, peak yield (Ymax), days in milk at peak yield (b/c), persistency (s), LACTATION length (LL) and the 305-day milk yield. The incomplete gamma function (Wood function) was used to estimate LACTATION CURVE and LACTATION PARAMETERS from daily milk records. Among the 100-day milk yield periods, the second 100-day milk yield had the highest herita-bility (0.29±0.024) and the highest genetic correlation with the 305-day milk yield (0.996±0.00). LACTATION CURVE PARAMETERS had low h2 (0.017±0.007 to 0.051±0.011). The b / c had a relatively high genetic correla-tion with the 305-day milk yield (0.52±0.08), a moderate genetic correlation with CI (0.32±0.14) and nega-tive genetic correlations with measures of somatic cell count. This suggested that b / c could be used as a criterion to improve 305-day milk yield and resistance to subclinical mastitis.