Flax (Linum usitatissimum L.) is one of the promising oil crops in the north of Kazakhstan. Over the last 10 years, the total area under this crop in the region has increased fourteen-fold, since flax is a very plastic crop for steppe and dry steppe conditions, and oil seeds are in high demand in the world market. Flaxseed oil, due to the content of polyunsaturated fatty acids, occupies one of the first positions among other edible vegetable oils. Depending on the environmental conditions, the oil content of the crop may vary from 36.4% to 52.0%, while, as noted by many researchers, the change in the oil content depends on the genotypic characteristics. Therefore, the study aims to analyze the influence of the genotype and climatic conditions of cultivation on the fat biosynthesis in flax seeds in the context of the dry steppe zone of Northern Kazakhstan on dark chestnut soils of the Akmola region. Field experiments were conducted in 2015–2017 with the study of nine oil flaxseed cultivars (of mid-season varieties) sown on May 20th at the seeding rate of 7 million seeds/ha based on the traditional technology of cultivation recommended for the region. The climatic parameters were taken into account according to the meteorological service data. The fat content was determined by the extraction method using a Soxhlet apparatus, in accordance with GOST (All-Union State Standard) 10857–64, and the moisture content was determined by means of the thermogravimetric method, according to the National Standard of the Republic of Kazakhstan 2.195–2010, for the recalculation of the oil content for absolutely dry matter. It was found that the formation of oil depends on the climatic parameters and on the sum of active temperatures during the ripening period, in particular. The evaluation of plasticity and stability of the cultivars (genotypes) showed that VNIIMK 620, Lirina, Karabalyksky 7 can be considered highly valuable cultivars and Severny, Biryuza, Kazar, Ilyich – valuable cultivars in terms of the oil content for the dry steppe zone.