Soil arsenic (As) contamination is a worldwide concern, and many methods have been applied to remediate arsenic contaminated soil. Despite iron oxides nanoparticles have been successfully used to immobilize arsenic in soil, the transformation and distribution of arsenic species in soil at presence of iron oxide nanoparticles are still limitations, practically in As (III) and As (V). Hence, in this study, firstly, iron oxide nanoparticles synthesized using euphorbia cochinchinensis leaves extract (GION) were used to immobilize total arsenic in a contaminated soil. Results indicated that the As were immobilized effectively in GION treatments. After 30, 60 and 120d incubation, the available (0.05 M (NH₄)₂SO₄ extractable) As decreased by 14.3%, 25% and 50%, respectively. The availability of As decreased significantly after 30, 60 and 120d incubation. Secondly, As species distribution is understood. The results indicated that As(V) was the main valence state in five soil fractions after long time incubation. Additionally, soil samples were characterized using characterization techniques. The result of X-ray fluorescence spectroscopic (XRF) showed that the Fe₂O₃ increase significantly in 9% GION treatment, X-ray photoelectron spectroscopy (XPS) showed that As(V)-O was the dominant valence state in the 0.05 M (NH₄)₂SO₄ extractable and residual fraction, whereas only As(III)-O was detected in the 0.2 M NH₄-oxalate extractable fraction in both control and GION treatments. However, in contrast to the control, As(V)-O decreased significantly (p<0.05) in the GION treatment. Overall, As in contaminated soil can be effectively immobilized by GION and the efficiency increased with incubation time. Finally, the result also indicated GION may cause the variation in As species, which should be further studied.