In this research, mathematical models describing lactation curve of Norduz sheep breed were compared, and lactation parameters were estimated by selected model describing the lactation curve better than others. Then, factors affecting the lactation parameters and lactation milk yield were investigated. In the analysis, a total of 2.366 test-day records belonging to 189 completed lactations were used. It was observed that the lactation curve showed a decrease after lambing followed by an increase towards to the middle of the lactation, and then a decrease afterwards, like a sinusoidal shape. The forth degree Legendre polynomial model (LEG4), comparing to other models used, showed the best performance in describing the lactation curve of Norduz sheep raised in farmer condition. Total lactation milk yield, corrected to 180-day lactation lenght, was 130.9 +/- 3.24 kg. It was found that parity, lambing age and lambing month have a significant effect on lactation milk yield (LSV), and lambing month and parity also have a significant effect on lactation trajectory.