Research Highlights: The correlations between the analyzed physical properties of seeds and seed mass were determined. The results were analyzed to determine most effective seed separation devices for the evaluated fir species. Background and Objectives: Information about the variations and correlations between the physical properties of seeds is essential for designing and modeling seed processing operations such as seed separation. The aim of this study was to determine the range of variations in the basic physical properties of seeds of selected fir species, and to identify the correlations between these attributes for the needs of the seed sorting processes. Materials and Methods: Terminal velocity, thickness, width, length, the angle of external friction and mass were determined in the seeds of 11 fir species. The measured parameters were used to calculate the geometric mean diameter, three aspect ratios, sphericity index and the specific mass of each seed. Results: The average values of the basic physical properties of the analyzed seeds were determined in the following range: terminal velocity – from 4.8 to 7.1 m s-1, thickness – from 1.76 to 3.22 mm, width – from 3.29 to 5.57 mm, length – from 5.44 to 11.06 mm, angle of external friction – from 26 to 33°, and mass – from 7.9 to 48.3 mg. The seeds of Sierra white fir where most similar, whereas the seeds of balsam fir differed most considerably from the seeds of the remaining fir species. Conclusions: Fir seeds should be sorted primarily with the use of mesh sieves with longitudinal openings to obtain fractions with similar seed mass and to eliminate the need for dewinging.