Climate changes inevitably affect plant production, but it is difficult to predict their direct impact on a vitality (germination) of a produced seed. Germination is the most important indicator of seed vitality and a key stage in the life cycle of plants. Seed germination is significantly affected by environmental factors during the growing season, as well as the method of processing (according to seed size). The aim of this work was to determine the impact of three factors on seed germination of four sunflower inbred lines: i) genotype, ii) environment conditions in a production year, and iii) seed size obtained during processing. The germination was examined in a standard laboratory test by ISTA Ruls, six months after the processing and natural breaking of dormancy. Larger and smaller sizes seeds, obtained in two consecutive years, which were characterized by different environmental conditions (sum of precipitation, mean monthly temperatures and air humidity during flowering), were used. Collected data was analyzed using a three-way analysis of variance, and correlation analysis. The coefficient of variation was 8.19%. Significantly higher average seed germination was obtained during 2020 (92%,) compared to 2021 (85%). The effect of seed size was not statistically significant, regardless of the tested inbred line. It can be concluded that the increase in the seed germination of sunflower inbred lines is mainly influenced by genotype, but also the increase in precipitation, minimum and maximum air humidity during flowering and the decrease in mean monthly temperatures, which was confirmed by the correlation coefficients between the above agroecological conditions and the germination rate of the produced seeds.