Dicer is responsible for producing small interfering RNA and microRNA as an essential component of RNA interference. The character of these small RNAs is determined by the structure of Dicer protein, which contains RNA HELICs, PAZ, RNaseand dsRNA binding domain. Different organisms have different numbers of Dicers with distinct but redundant functions. Evolutionally, the amount of Dicer in organism decreases, but its functions become multiple and extremely important. Studies on structure and function of Dicer will improve our understanding of the mechanism, as well as the evolutional rule and character, of RNAi and relative pathways. Here, we review the structural and functional character of Dicer.