The objective of resent study was to investigate environmental factors affecting milk somatic cell counts and estimate the variance components and genetic and phenotypic trends for somatic cell score in Holstein dairy cattle of Iran. This research was conducted in a large scale by using dataset collected by Animal Breeding Center of Iran. The dataset consisted of 4,279,953 test day records of somatic cell counts (SCC) gathered from 2002 to 2013 on 1281 dairy farms in Iran. All of statistical analysis was performed using different procedures (Procs GLM, Reg and Univariate) of SAS software by including somatic cell score (SCS) as the dependent variable. To estimate the genetic parameters and variance components, an animal test day repeatability model was used by the ASREML software. The Result of this study indicated that All factors in the model including herd, parity and year of calving, month of recording, milk yeild, days in milk (DIM) and age at first calving (AFC) had significant effects on SCS (P 0.01) and by increasing parity number, SCS increased. The lowest and highest levels of SCC were observed in months of Bahman (21 January-19 February) and Mehr (23 September-24 October), respectively. There was a linear relationship between AFC and SCS, so that by increasing AFC, the SCS increased. As daily milk yield levels increased, the SCC has reduced. Obtained results in the research revealed that pattern of phenotypic trend for SCS through lactation were inverse of milk lactation curve i.e. as DIM increased, SCC reduced. The pattern of phenotypic trend for SCS through investigated years, was descending. The heritability was estimated to be 0.176. Additive, phenotypic and residual variance componenets were 0.639, 3.622 and 2.210, respectively. Repeatability (± SE) within (r wit ) and between (r acr ) pariaies were estimated to be 0.362 ± 0.006 and 0.230 ± 0.002, respectively. The genetic trend, calcauted based on regresion of avarage breeding values of SCS on year of birth, was -0.0246.The phenotypic trend was -0.047. This trend can partially be due to genetic improvement and mastitis management control programs that are widely being used across the country dairy farms. Keyword: somatic cell count, Holestein dairy cattle, test day record