Two field experiments were conducted on the Experimental Farm at El-Gemmeiza Research Station, Gharbia Governorate, Agricultural Research Center, Egypt, during two successive winter seasons of 2018/2019 and 2019/2020 to investigate the effect of three nitrogen fertilizer rates (30, 50 and 70 kg N/fed) and three plant densities (1500, 2000 and 2500 seeds/m²) on seed and oil yield and its related traits of three flax cultivars (Sakha 3 Giza 11 and Giza 12). The experimental design was laid out using split-split plot design in four replications. Flax cultivars were distributed in the main plots, whereas nitrogen fertilizer rates were arranged at random in sub-plots and plant densities treatments were assigned at random in sub-sub plots. The sub-sub plot area was 9 m². Giza 11 cultivar significantly recorded the maximum mean values in No. of upper branches/plant, No. of capsules/plant, No. of seeds/plant, seed index, seed yield/plant, seed yield/fed, harvest index, oil yield/plant and oil yield/fed in two seasons. Meanwhile, the maximum mean values of seed oil content in both seasons were obtained from Giza 12 cultivar. While, flax cultivar of Sakha 3 significantly gave the maximum upper branching zone length and No. of seeds/capsule in both seasons. All seed and oil yield and its related traits of flax were significantly increased with increasing nitrogen fertilizer rates from 30 to 50 and 70 kg N/fed under study in two seasons. Flax plants growing at lowest plant density (1500 seeds/m²) markedly gave the greatest mean values of upper branching zone length, No. of upper branches/plant, No. of capsules/plant, No. of seeds/plant, seed yield/plant, harvest index and oil yield/plant in both seasons. Meanwhile, the maximum mean values of seed yield/fed and oil yield/fed which were obtained from flax planting at highest plant density (2500 seeds/m²) in both seasons. The first order interactions between treatments Giza 11 X 70 kg N/fed, Giza 11 X 1500 seeds/m² and 70 kg N/fed X 1500 seeds/m² as well as the second order interaction between treatments Giza 11 X 70 kg N/fed X 1500 seeds/m² were significantly recorded the greatest seed and oil yield of individual plants as compared with the others interactions in both seasons. While, the maximum mean values of seed and oil yield/fed were recorded from the first order interactions between treatments Giza 11 X 70 kg N/fed, Giza 11 X 2500 seeds/m² and 70 kg N/fed X 2500 seeds/m² as well as the second order interaction between treatments Giza 11 X 70 kg N/fed X 2500 seeds/m². It could be concluded that flax planting cultivar of Giza 11 and fertilizing by 70 kg N/fed with plant density of 2500 seeds/m² to maximizing seed and oil yield/fed.