Genetic trend for production traits were estimated in Iranian Holstein cattle using the first lactation data recorded by Cattle Breeding and Dairy Product Improvement Center of the State from 1991 to 2006. Effect of some factors on genetic trend was investigated using data from 18 large herds. (Co) variance components and genetic parameters were estimated, using five trait animal model and EM-AI algorithm in WOMBAT software. Estimated heritability for milk, fat, protein yields, fat percent and protein percent were 0.255, 0.197, 0.210, 0.317 and 0.308, respectively. Linear regression of Estimated Breeding Values (EBV) on birth year was used to estimate genetic trend. Estimated genetic trend for the animals for milk, fat and protein yield were 35.437±2.655, 0.714±0.059, 0.945±0.069 kg and for fat and protein percent were -0.006±0.0005 and -0.002±0.0002, (P<0.001). A comparison of genetic trend among 18 herds indicated significant difference among various herds (P<0.001). An investigation of some reproductive factors on genetic trend showed average ages at first calving (P<0.01) and mean of parities (P<0.05) exhert significant effects on the genetic trend. Effect of culling on genetic trend of production traits was significant (P<0.05). Animals were mainly culled involuntarily in herds. The impact of involuntary culling on genetic trend was significant (P<0.01).