MicroRNAs (miRNAs) are a type of small non-coding RNAs that regulate cognate mRNA expressions at the post-transcriptional stage. Although several miRNAs are known to be involved in various biological processes, including developmental timing, patterning, embryogenesis, differentiation and organogenesis, growth control, and apoptosis, many target genes and the functions of most miRNAs are still unclear. Since there is only a partial complementarity between miRNAs and their targets in animal cells, it is difficult to identify the specific target genes for a given miRNA and elucidate its function. In this study, we confirmed that bone morphogenetic protein receptor II (BMPRII) is a direct target of miR-21, and also showed that the protein level of BMPRII correlates inversely with the amount of miR-21 in PC3 and Lncap cells. These findings suggest that miR-21 may have a potential role in regulating the malignancy and metastatic abilities of prostate cancer cells and in self-renewal of stem cells by regulating the expression of BMPRII.