The present study was carried out to investigate the persistency of lactation milk yield and effects of some environmental factors on this characteristic in Brown Swiss cattle in Altinova State Farm. The same feeding and management program was applied to all the cattle. In the study, in terms of measurable environmental factors; the effects of lactation turn, year and season were determined. In the statistical analysis of data, least squares method for the determination of effects of environmental factors and contrast test for the comparisons of data were used in GLM procedure. In the Brown Swiss cattle, the results of persistency of lactation milk yield, which were estimated with P2:1 , P3 : 1 and P3 : 2 models, were 91.93%, 59.91% and 64.46%, respectively. The effects of lactation turn and year on persistency of lactation estimated with P2 : 1 and P3 : 1 models (P<0.001), and lactation turn and season estimated with P3 : 2 model (P<0.001 and P<0.05) were found to be significant. Because of higher persistency level of young cows, by keeping young cows at their peak of lactation for longer intervals in the herd, the milk productivity of herd can be increased.