MicroRNAs (miRNA) are small noncoding RNAs that posttranscriptionally regulate expression of target mRNAs. In animals, miRNAs control many developmental and physiological processes. It has recently revealed that miRNAs critically work in B cell development and some B cell malignancy and that their expression profiles well correlate with its prognosis and phenotype. This review overviews these interesting findings and discuss a possibility that miRNA is a promising target for therapeutic and diagnostic tool for B cell malignancy.