Objective: To investigate the effect and possible mechanism of microRNA-504 (miR-504) in proliferation and apoptosis of non-small cell lung cancer (NSCLC) cells. Methods: Expression of miR-504 in three NSCLC cell lines (A549, H1299 and HCC827) and control human bronchial epithelial cell line BEAS-2B were determined by real-time PCR. The cell line (HCC827) with the most significant change of miR-504 expression was used in the following study. Then HCC827 cells were transfected with miR-504 mimic, miR-504 inhibitor and non-targeting miR-control by using Lipofectamine RNAiMAX. Cell proliferation of the three above-mentioned groups was detected by CCK-8 kit. Cell apoptosis of the three groups was measured by Annexin V-FITC/PI apoptosis detection kit. Expression of P53 of the three groups was determined by Western blot. Results: Compared with BEAS-2B control cells, expression of miR-504 was significantly increased in all three NSCLC cell lines. HCC827 cells were used in the following study due to its most significant increase of miR-504. Compared with miR-control group, over-expression of miR-504 (miR-504 mimic) resulted in accelerated proliferation and decreased apoptosis of HCC827 cells. While down-regulation of miR-504 (miR-504 inhibitor) led to retarded proliferation and increased apoptosis. In addition, expression of P53 was decreased in miR-504 mimic group but increased in miR-504 inhibitor group. Conclusion: miR-504 could promote proliferation and inhibit apoptosis of NSCLC cells, which was possibly mediated by down-regulating P53 expression.