In this study, a commercial insecticide, Altach 5EC, has been used to investigate its cytotoxic effects on chromosomes and mitosis of cells in the root tip of Allium fistulosum L. The result indicated that Altach 5EC which contaning two different concentrations of alpha- cypermethrin (0,047 mg/ml and 0,062 mg/ml) were used to treat the root tips for 8 hours led to a decrease in the mitosis index compared to the negative control. Especially, various types of chromosomal mutations were observed such as broken chromosomes, chromosomal bridges, and slow-moving chromosomes. At the same time, numerous disturbances of mitosis were observed, which added the data for the practice lesson of Mitosis. These disturbances included sticky chromosomes, metaphase disturbance, fault polarization in anaphase and telophase, and sticky metaphase. The study revealed that Altach 5EC is a suitable commercial insecticide for effectively treating the roots of Allium fistulosum L. in both the “Chromosomal Structural Mutations” and “Mitosis” lessons within Genetics - Evolution subject at Biology Department, Vinh University.