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The cellular and molecular toxicity effect of Aloe Vera crude extract on meristem cells of onion (Allium cepa) roots were studied . Different concentrations of this crude extract 20% ,10% ,5% ,2% ( and 40%) were used at different periods ; 24 ,48 and 72 hour . Number of chromosomal aberrations and the mitotic index was calculated . In addition, the genetic effects were observed by using randomly amplified polymorphic DNA ( RAPD ) technique . The results showed that the gel crud extract of Aloe vera had inhibition effects on the growth of the onion roots with 10% EC50 value. Significant effects of this gel crud extract were also observed on the chromosomal aberration and mitotic index by decreasing of divided cell rates. The toxic effect of 20% concentration of gel crud extract was decreased the mitotic index under 50% , in the comparison with control sample . Chromosomal aberration were also observed in divided cells of onion roots, which exposed to different concentrations at different periods . The higher frequent type of this chromosomal aberrations , which observed are stickiness , disturbed , bridge , c-mitosis , vagrant and star telophase . The result of RAPD technique reveled differences in number of DNA bands and their molecular weights of the treated samples , which exposed to high concentration of crud extract in comparison with the control sample . The result of the genetic stability ratio (% GTS), also showed that Aloe vera crud extract have genetic toxicity at 40% concentration.