Recently, a class of ～21 nucleotides(nt) small RNA have been discovered in many eukaryotes, termed miRNAs (microRNAs), which were first identified as key temporal regulators in development. So far, large quantities of studies have revealed that miRNAs have played important roles in genetic control at many different levels and rearrangement of genome. Besides, its association with siRNA (small interfering RNA) previously discovered in RNAi (RNA interference) in the further researches becomes much closer than it has ever been considered. Its pathway directing translational repression, the surprisingly high conservation of certain miRNA, the mechanism of process of mature miRNA and genetic regulation compared with that of siRNA were focused. Finally several discussions arising people's interests caused by the discovery of miRNA are made.