It has been demonstrated that microRNA (miRNA) and small interfering RNA(siRNA) play an important role in the basically physiological regulation of eukaryotic organisms. With the intensive study on siRNA and miRNA, scientists recently have discovered that there exists a novel type of small RNA named as piRNA in the mammalian testes, which is essential for physiological modulation of spermiogenesis and different from miRNA or siRNA in functions, gene distribu-tions and characteristics. It is well believed that the more intensive research on piRNA will throw light on the delicate mechanisms of gene expression modulation for eukaryotic species.