This study was aimed to explore the regulation of arsenic trioxide (As₂O₃) on imbalance between adipogenic and osteogenic differentiation of BM-MSC from patients with aplastic anemia(AA). The BM-MSC from AA patients were separated and purified, placed into the adipogenic and osteogenic differentiation culture system, then added the As₂O₃, CsA, As₂O₃combined with CsA were added to corresponding differentiation culture system, the concentration of As₂O₃and CsA were set at 0.001 µmol/ml and 2.5 mmol/ml respectively, the cells were divided into As₂O₃group, the CsA group, combined group and control group (no drug). The cell morphological observation, oil red 'O' staining, Von-Kossa staining, and RT-PCR were used to detect corresponding differentiation marks. The results indicated that in respect to adipogenic differentiation, cellular morphology observing and oil red 'O' staining showed that the rate of adipocyte differentiation in As₂O₃group was (18.3 ± 1.9)%, which was lower than the (91.8 ± 2.7)% in the CsA group and (92.1 ± 1.2)% in control group (P < 0.05), there was no significant difference in comparison with (8.3 ± 1.9)% in the combined group (P > 0.05), but the rate of differentiation in CsA group was higher than that in combined group (P < 0.05), and there was no significant difference in comparison wtih control group. RT-PCR showed that the LPL-mRNA expression level in As₂O₃group were significantly lower than that in the CsA group and the control group (P < 0.05), no difference was observed while compared with the combined group (P > 0.05), but the LPL-mRNA expression level in CsA group was significantly higher than that in the combined group (P < 0.05). In terms of osteogenetic differentiation, the calcium deposition in As₂O₃group and combined group was obviously observed while rarely in the CsA group and the control group when detected by the Von-Kossa staining. OST-mRNA expression level in As₂O₃group were higher than that in CsA group and the control group (P < 0.05), while compared with the combined group, there was no significant difference (P > 0.05), but the OST-mRNA expression level in the CsA group was lower than that in the combined group (P < 0.05). It is concluded that As₂O₃can significantly enhance the ability of BM-MSC from AA patients to differentiate into osteoblast, also can inhibit the adipogenic differentiation, in contrast, the CsA can not promote the osteoblast differentiation of BM-MSC from AA patients.