Background: MicroRNAs (miRNAs) are involved in the initiation and development of cancer, and participate in drug resistance. Paclitaxel (PTX) is rst-line chemotherapy drug for advanced non-small cell lung cancer (NSCLC). The abnormal miRNA expression in NSCLC and its association with chemotherapy drug resistance remains largely unknown. The study aimed to investigate the aberrant expression of miR-221-3p in NSCLC and to elucidate its molecular mechanisms in relation to PTX resistance. Methods: Quantitative polymerase chain reaction (qPCR) was used to examine miRNA and messenger RNA (mRNA) in NSCLC tissues and cell lines. The roles of miR-221-3p in NSCLC progression and drug resistance were assessed by Western blot analysis, colony formation assay, and CCK-8. Dual-luciferase reporter assay was conducted to evaluate the interaction between miR-221-3p and MDM2. Xenograft tumor models were established by subcutaneously injecting PTX-resistant A549 cells (A549/Taxol) to assess the effect of miR-221-3p on tumor growth. Data regarding miRNAs and target proteins from 20 NSCLC tissues and paired non-cancerous matched tissues were also obtained for correlation analysis. Results: PTX increased miR-221-3p expression, which regulated MDM2/P53 expression in the PTXsensitive NSCLC strain (A549). Meanwhile, miR-221-3p was rarely expressed and not interfered by PTX in A549/Taxol. Dual luciferase reporter assay con rmed that miR-221-3p speci cally binds to MDM2 mRNA. MiR-221-3p down-regulation reduced the sensitivity of A549 cells to PTX, whereas its up-regulation partially reversed the A549/Taxol cells resistance to PTX and increased the chemosensitivity of A549/Taxol cells to PTX in xenograft models. QPCR analysis revealed that miR-221-3p expression increased, whereas the MDM2 level decreased in NSCLC tumor tissues. Furthermore, the result of Western bolt analysis showed that P53 was lowly expressed in tumor tissues with MDM2 overexpression. Conclusions: MiR-221-3p overexpression could regulate MDM2/p53 signaling pathway to reverse the PTX resistance of NSCLC and induce apoptosis in vitro and vivo.