The miR-17-92 cluster has long been recognized as an oncogenic microRNA (miRNA) cluster and is amplified in multiple cancers. However, the individual roles of its members in carcinogenesis are largely undetermined. After transfection of miR-18a mimics, an antisense oligonucleotides inhibitor, siRNAs and a luciferase reporter plasmid, the MTT assay, colony formation assay, semi-quantitative RT-PCR, luciferase assay and Western blot analysis were conducted in bladder cancer cells. In the present study, we showed that miR-18a, a member of the miR-17-92 cluster, suppressed cell proliferation in bladder cancer T24 cells. Furthermore, ectopic expression of miR-18a in T24 cells down-regulated Dicer expression at both the mRNA and protein level, while inhibition miR-18a by antisense oligonucleotides could enhance Dicer expression in T24 cells. Two binding sites of miR-18a were found in Dicer 3' untranslated region (3' UTR). Luciferase reporter assay demonstrated that both sites could mediate expression suppression in vitro. In addition, knockdown of Dicer expression by siRNA mimicked cell growth suppression induced by miR-18a in T24 cells. These results show that miR-18a functions as a tumor suppressor by targeting Dicer in bladder cancer T24 cells and revealed a noteworthy feedback loop, which may be utilized by the miR-17-92 cluster to control miRNA output and prevent its overexpression.