Abstract This research paper investigates the influence of the dyeing process on the strength of wool fibers, specifically focusing on the use of reactive black dye. The study involved testing and measuring the average break in extension and breaking load of undyed, blank, and dyed samples with various average diameters. To assess the strength of fibers fifty separate fibers were selected for each sample group. The findings indicate that thinner fibers exhibit reduced breaking extension and lower breaking load compared to thicker fibers. Moreover, the average breaking elongation and breaking load of both fiber types decrease following the dyeing process. The results also indicate that as fiber diameter decreases, the decrease in breaking extension becomes more pronounced, while as fiber diameter increases, the decrease in breaking load becomes more pronounced. Notably, these results align with theoretical predictions and represent a significant advancement compared to previous studies. Key words: Dyeing Process, Strength, Wool Fibers, Black Color, Reactive Dye.