Performance of one genotype in similar climates is approximately the same but if this genotype is introduced into a different climate, its performance will be affected, based on Nizamani and Berger (1996). The function that relates phenotype to environment is unique for each genotype. Hence, the response to changes in environment may vary from one genotype to the other, based on Mulder et al. (2004). Different selection responses between environments are generally attributed to two types of genotype by environment interaction. The first type occurs when the genetic correlation between performances in two environments is substantially less than 1.0, indicating a genetic difference basis for the trait in the two environments. The second type of genotype by environment interaction results from heterogeneous variances, based on Ojango and Pollott (2002). The goal of this study was to estimate genetic (co)variances, environmental variances, and genetic parameters of milk production and fat yield among different environments in Iran to determine variables that are useful indicators of genotype by environment interaction.