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In this research a random regression test day model was used to estimate genetic parameters and genetic trend for 305-day milk yield in Iranian Holsteins. A total of 171360 monthly test day milk records obtained from 34257 first lactation Iranian Holstein cattle (three times a day milking) distributed in 96 herds and calved from 1999 to 2008, were used. The fixed effects of province-herd- year-season of test day as contemporary group and age at calving and Holstein gene percentage as covariates were fitted in the model of analysis. Orthogonal Legendre polynomial up to order 3 was implemented to take account of genetic and environmental aspects of milk production over the course of lactation. The results showed that average of heritability estimates for the second half of lactation period was higher than that of the first half. The heritability estimate for the first month was lowest and for the eighth and ninth months of the lactation were the highest compared to the other months of lactation. The genetic correlations between monthly test day records decreased by increasing the distances between months of lactation. Linear regression of estimated breeding values on calving year was used to estimate genetic trend. Estimated genetic trends for sires and dams for milk yield were 25.13 and 15.45 kg per year, respectively. In this research, genetic trends were estimated by a random regression test day model; therefore using this method is the exact way to take account of genetic trend rather than the other ways.