Background: MicroRNA-21 (miR-21) was previously reported being dysregulated in many kinds of cancer including esophageal squamous cell carcinoma (ESCC). In the present study, we aimed to investigate the role of miR-21 in ESCC, especially in its effects on radiation-sensitivity of ESCC. Methods: Expression of miR-21 was detected in 63 pairs ESCC tumor and adjacent non-tumoral tissues using qRT-PCR, correlation between miR-21 and clinicopathological feature of ESCC was analyzed. The role of miR-21 in the proliferation, cell cycle and apoptosis of ESCC cells during irradiation were studied. Results: MicroRNA-21 expression was significantly increased in ESCC tumor tissues. Expression of miR-21 was positively associated with advanced clinical stage. Under irradiation, overexpression of miR-21 increased cell proliferation and cells in S phase, and inhibited cell apoptosis of ESCC cells. In contrast, knockdown of miR-21 had an opposite effect. Conclusions: Downregulation of miR-21 inhibited the radiation-resistance of ESCC, whereas overexpression of miR-21 increased the radiation-resistance. MiR-21 is a potential novel target for developing specific treatment interventions in ESCC in future.