The cellular response to ionizing radiation exposure is very complex and involves many pathways. A wide variety of biologic effects are induced after exposure to ionizing radiation, ranging from DNA damage processing, signal transduction, mutations, altered gene expression, cell-cycle arrest, genomic instability, and induction of carcinogenesis to cell death. To gain insight into this complex response, global alterations in the expression of genes in irradiated cells have been examined. Recent studies have provided evidence to associate micro-RNA (miRNA) with many cellular processes, including carcinogenesis, timing of cell-fate decision, apoptosis, and metabolic pathways controlling a range of events. The small noncoding miRNA are emerging as critical components in controlling the gene expression. Because miRNA target so many genes, we hypothesized that alterations in their expression may be associated with the overall response of cells to radiation treatment. To explore the role of miRNA in cellular response to ionizing radiation, we monitored the expression levels of several miRNA by employing the stem-loop real-time polymerase chain reaction in Jurkat and TK6 cells treated with gamma-radiation. The expression levels of several members of the let-7 family miRNA that functionally inhibit the mRNAs of Ras oncogenes were upregulated after ionizing radiation treatment in Jurkat cells but were downregulated in TK6 cells. The expressions of miRNA associated with MYC translocation were upregulated in both cell types. The modulation of miRNA involved in various cancers was also examined. These results provide a first glimpse to indicate the involvement of miRNA in radiation-induced stress response and will lead to functional studies dissecting the molecular details of these processes.