Arsenic trioxide has shown remarkable biological activity against bladder cancer in some clinical studies. However, the mechanism of its action is unknown. Our aim was to find the relationship between miRNAs and arsenic trioxide treatment by using T24 human bladder carcinoma cells. By performing microRNA microarray and quantitative real-time PCR after ATO treatment, we found that expression levels of several miRNAs, in particular, miRNA-19a, were significantly decreased in T24 cell line. Furthermore, cell proliferation assay, flow cytometry analysis, prediction of miRNA targets, Western blot analysis, and luciferase reporter assay were performed to determine the role of mir-19a in affecting the biological behaviors of T24 cells. Several miRNAs were up-regulated or down-regulated in T24 cells treated with arsenic trioxide compared to their controls. If only changes above two folds were considered, two miRNAs were identified, miRNA-19a was down-regulated, while miRNA-222* was up-regulated. Among them, knockdown of miRNA-19a by anti-miRNA-19a transfection showed a positive therapeutic effect in bladder cancer cells by inhibiting cell growth and inducing cell apoptosis targeting PTEN through the PTEN/Akt pathway. Besides this, a synergy effect was detected between knockdown of miRNA-19a and arsenic trioxide. Arsenic trioxide altered miRNA expression profile in T24 cells. It seems miRNA-19a plays a critical role in the mechanism of arsenic trioxide treatment in bladder cancer. The synergy effect between miRNA-19a and arsenic trioxide that advocates targeting the mir-19a may represent a potential approach to enhance the efficacy and safety of ATO to treat bladder cancer by a decrease in dose.