To determine arsenic (As) phytotoxicity in gramineae, a hydroponic experiment with oat was conducted. Twenty-four days old oat seedlings were treated with sodium meta-arsenite (NaAsO2) for 14 days at different rates of As in the greenhouse. Shoot growth decreased by 28.17, 38.34, 68.34, and 75.85%, while the root growth decreased by 33.51, 42.61, 64.79, and 72.15% for 6.7, 13.4, 26.8, and 53.6 µM As treatments, respectively, indicating that the roots were more sensitive to As-toxicity than that of shoots. Arsenic at the rate of 13.4 µM level produced chlorotic symptom in the 4th leaf. Necrotic symptom was found in the old leaves at 26.8 and 53.6 µM As levels. Arsenic concentration increased both in shoots and roots with the increasing As concentration in the solution. Mostly, the concentrations of P, Fe, and Zn decreased in shoots. On the contrary, As and Fe concentrations increased in roots with the increasing As concentration in the solution. The research suggested that As hindered Fe translocation from roots to the shoots, resulting in whitish chlorotic symptom in the leaves.