Background: Realgar, an arsenic tetrasulfide compound, is a highly recognized traditional Chinese medicinal prescription that has been widely used to treat various diseases such as inflammatory diseases. However, there are still some problems in the clinical treatment of Realgar, such as large oral dose and high potential toxicity. Objective: To evaluate effects of Realgar nanoparticles on lupus nephritis (LN) in vivo in MRL/lpr mice. Methods: Ten-week mice were orally administered every day for eight consecutive weeks except the mice of normal model groups. The serum levels of anti-ds-DNA antibody IgG, IgM, IFN-γ, Creatinine (Cr), and blood urea nitrogen (BUN) were determined, and 24-hour urine protein was also measured. Renal inflammatory pathology analysis was assessed by hematoxylin-eosin (H&E) staining. The expression of phosphorylated signal transducer and activator of transcription 1 (p-STAT 1) and Janus Kinase 1 (JAK 1) in kidney tissue was determined by direct reverse transcriptase-polymerase chain reaction (RT-PCR) and immunohistochemistry (IHC). Results: The mice treated with Realgar nanoparticle in the high dose-treated (Realgar HD, 0.03 g/kg/d) group exhibited significantly reduced serum levels of anti-dsDNA (p<0.01), IgG (p<0.01), IgM (p<0.01), BUN (p<0.01), Cr (p<0.01), and inflammatory cytokine IFN-γ (p<0.01) as well as proteinuria (p<0.01) compared to the untreated model MRL/lpr mice. Additionally, high doses of Realgar nanoparticles significantly suppressed the phosphorylations of STAT 1 (p<0.01) and the renal pathological changes. Conclusions: The study indicates that Realgar nanoparticles may be a potential agent to treat LN, and the down-regulated p-STAT1 expression suggests that it may be one of the LN treatment targets for Realgar nanoparticles.