The objective of this study was to evaluate the phenotypic lactation curve of crossbred dairy cows raised in Phetchaburi and Prachuap Khiri Khan Provinces. Monthly sampling of daily milk yield of 5516 records from 485 first lactating heifers collected by the Biotechnology in Livestock Production Division Department of Livestock Development during the year 1997 to 2003 were used in this study. Monthly sampling of daily milk yield were fitted using Wilmink function (yt=a+bt+cexp-0.05t) to describe the whole and individual curves and estimated lactation parameters (a b and c). All parameters were used to calculate milk yield at day 60 (MY60) 280 (MY280) peak yield (PM) and accumulated 305 days yield (MY305). The results showed that the fixed effects of herd-year-season of calving level of HF blood or genetic provinces and age of calving were significant differences of all traits (P<0.05; a b c MY60 MY280 PM และ MY305). The phenotypic correlations between the considering traits were significant differences (P<0.05). Moreover the study on the nature of lactation curve could describe the lactating abilities and it could represent the others important traits in lactation curve. Therefore these results could guide the farmers to manage their farm and feeding the cattle to be more effective.