The kit applies improved guanidine thiocyanate-phenol one-step method to lyse samples and inactivate ribonucleases，then genomic DNA and 18 and 28s RNA are removed by the first spin-column. Then microRNA (including miRNA, snRNA,and other RNA less than 200bp) is absorbed by the second spin-column. After a serial of elution-centrifugation steps to remove cellular metabolite and proteins etc, finally microRNA will be eluted from silica membrane using low salt RNase-free water.