This experiment was carried out to investigate the effects of mercury and arsenic on the growth of Arabidopsis thaliana when treated with three different concentrations. When treated with mercury, there was no noticeable difference in the growth of the plant between the group treated with $0.5;{mu}g/L$ (the effluent standard established by the Ministry of Environment) and the group treated with the concentration 100 times higher. They both showed almost the same level of growth as that of the normal plant. But the group of the concentration 10 times higher showed significantly $10\%$ more growth compared with the normal plant. When treated with arsenic, the three different groups all showed a little more growth compared with the normal plant. Interestingly, the group of the concentration 10 times higher than the official standard concentration of arsenic $(50;{mu}g/L)$ showed the highest level of growth, significantly $20\%$ more than the normal plant. These results show that some amount of mercury and arsenic in the soil do not have much effect on the growth of Arabidopsis thaliana, and that optimum concentrations of mercury and arsenic can even stimulate the growth of the plant.