Tannery effluent contributes a significant role to increase pollution in the environment; especially it contains toxic heavy metals which cause toxic effect on plant genetic materials. Among tannery effluent chemicals chromium and lead have cytotoxicity and genotoxicity potentials on Allium cepa. This investigation was undertaken to assess the physico-chemical properties of tannery wastewater and their effect on the genetic materials of Allium cepa. Effluents physico chemical characteristics were undertaken using digital instruments for direct measurement and standard methods of atomic absorption spectrophotometer, colorimetric analysis. A series of six onion bulbs were grown in 0%, 20%, 40%, 60%, 80% and 100% concentration of wastewater (v/v) ratio and root tips from each onion bulb were cut and processed for analysis by Aceto-orcein squash technique. Most effluent components were above the discharge limit standards set by Federal Environmental Protection Agency and Ethiopian Environmental Protection Authority. The cytotoxicity effect on the root growth showed a significant reduction at high concentrations. Simple regression analysis showed that the result of mitotic index were statistically significant (P<0.05) in different concentration. A decrease in mitotic index with increase concentration of the effluent was observed. The effluent had induced chromosomal abnormalities such as laggard, fragmentation, stickiness, bridge, micronucleus, binucleated and morphologically changed nuclei in Allium cepa root cells among others. The results showed that cytotoxicity, genotoxicity and chromosomal aberrations were induced by the tannery effluent. Industries shall think of biological waste treatment methods.