MicroRNAs (miRNAs), with the length of about 22 nucleotides, are a growing family of small non-protein-coding RNAs that function as post-transcriptional regulators of target genes, involved in multiple processes of life and closely related with tumorigenesis. Recently, some aberrantly expressed miRNAs have been discovered in the prostate, indicating that miRNA may play a critical role in the molecular mechanism of the pathogenesis of prostate cancer. With deeper studies on human serum and plasma miRNAs, serum miRNA is promising to be a potential diagnostic biomarker for prostate cancer.