The persistency of dairy milk yield, which refers to the degree to which milk yield is maintained from month to month by the cow during her lactation, is an economic trait. The knowledge of persistency is important for herd management and selection strategy. The benefits of selecting for lactations that are more persistent were only speculative, because very little research has been published. As the time goes by, the more is learned about this trait and its relationship with other important traits. The persistency can be defined in several ways, as either ratios of yield in different parts of lactation period, to be derived from factors in lactation curves models as proposed by Wood (1967) or a simple statistical parameter computed from test-day yield records(3). The incomplete gamma function, which described by Wood is one of the most popular models used to describe the lactation curve: yt = atbe-ct where yt = milk yield on day t, a= a parameter to represent yield at the beginning of lactation, b and c are factors associated with the inclining and declining slope of the lactation curves, respectively. The objectives of this study were to estimate genetic and phenotypic parameters for different criteria of persistency, finding correlation between them and their relationships with 305-days milk yield, by applying REML under animal models in the Holstein dairy cattle of Iran.