Objective:To prepare magnetic poly D,L-lactide-co-glycolide oxymatrine nanoparticle(M-PLGA-OM-NP) and investigate its preventive effects against experimental liver fibrosis.Methods: The M-PLGA-OM-NP was prepared by multiple emulsion process and observed by transmission electron microscope(TEM).Dimethylnitrosamine (DMN)-induced liver fibrosis was established with mice.After intervention with M-PLGA-OM-NP,the expression of α-smooth muscle actin (a-SMA) in the livers of mice was detected by immunohistochemical assay and the score of liver fibrosis was determined by H-E staining and Van Gieson (VG) staining.Results: The prepared M-PLGA-OM-NP was in a regular sphere shape,with a mean diameter of 146.5 nm; the drug loading was 7.61% and the encapsulation ratio was 44.8%.The serum Alt level in the M-PLGA-OM-NP group was decreased compared with that in the model group.Light microscopy revealed that the fibrosis in the M-PLGA-OM-NP group was greatly improved compared with the model group and pure oxymatrine group.The expression of a-SMA,the marker of hepatic stellate cell activation,was obviously decreased in the M-PLGA-OM-NP group compared with that in the other groups.Conclusion: The prepared M-PLGA-OM-NP under magnetic field can reinforce the preventive effect of oxymatrine against DMN-induced experimental liver fibrosis in mice.