This research aimed to estimate heritability, phenotypic and genetic trends for the trait of energy-corrected test day milk of Iranian first-parity Holstein cows. A total of 774,013 test day recordson88,456 first-paritycows(three times milking a day) calvedduring 1997-2009 in 165 herds (six provinces) was used. Genetic analysis was carried out by a fixed regression test day model in which the effects of province, herd, calving year, production month, cow genotype (grade or pure Holstein), sperm type, production age, Ali and Schaeffer’s polynomial function, as well as additive genetic and permanent environmental random effects of the cows were included. Heritability and repeatability of the trait were found to be 0.127 and 0.421, respectively. Statistically significant phenotypic (0.414 kg) and genetic (0.076 kg) annual trends were found (P<0.0001). There was a significant difference (P<0.0001) between mean breeding value for the progeny of local (-0.031 kg) and exotic bulls (0.3747 kg). This research revealed that genetic trend for 305-d energy corrected milk trait was found to be 0.26% of the phenotypic mean indicating that selection pressure has not been too effective over the past years.