Altered miRNA expression could be a determinant of cancer development and/or progression. We aimedto study the role of oncomir miR-21 and tumor suppressor let-7a in the genesis of Opisthorchiasis-associatedcholangiocarcinoma (CCA). The results showed that miR-21 was up-regulated while let-7a was down-regulatedduring cholangiocarcinogenesis in the hamster model and also in human CCA samples. The expression levelof miR-21 had an inverse correlation with the mRNA level of its target RECK, a metastasis suppressor, inhuman CCA. Knockdown of miR-21 of KKU100 CCA cells significantly increased the mRNA level of RECKand suppressed the wound-induced migration of CCA cells. Our data suggest that miR-21 is one key moleculeplaying crucial roles in the CCA growth and metastasis. Manipulation of miRNA expression offers a potentialavenue of CCA therapy.