The present study aimed to explore the effect of oxalic acid (OA) different concentrations (1, 2 and 4 g/L) and Sclerotium cepivorum crude extract on the cytology of Allium cepa germinating roots during four durations 6, 12, 18 and 24 h. The rate of cell division and mitotic chromosomal behavior in root tips of germinated bulbs beside the numbers and types of chromosomal abnormalities were recorded for the selected samples during the period of treatments after 6, 12, 18, 24 h by oxalic acid 1, 2, 4 g, and S. cepivorum crude extract, The results showed that the mitotic indices were reduced when the roots were treated by different concentrations of oxalic acid and S. cepivorum crude extract, and the reductions are proportional with both concentration and time. Also, several chromosomal abnormalities were recorded that includes vacuolated nuclei, rod-shape, binucleated cells, disrupted nuclei at interphase and lagged chromosomes, bridge, polyploidy, mega cell, star metaphase, mini-chromosomes, ring chromosomes, and deformed nuclei during mitosis. Results suggested that the oxalic acid affects pH value, Ca⁺ depletion and plasma membrane maceration, and it induces programmed cell death in onion.