Background: Understanding the molecular basis underlying metastasis of non-small-cell lung cancer (NSCLC) may provide new therapeutic modality for the treatment of NSCLC. However, the mechanisms by which tumor-associated macrophages (TAMs) affect NSCLC metastasis still remain undefined.Methods: The role of macrophages in NSCLC was elucidated by gene set enrichment analysis via The Cancer Genome Atlas database (TCGA) database, and we further verified it through Quantitative real-time PCR and immunohistochemical staining. Exosomes from TAMs were extracted and co-cultured with A549 cells,the biological functions of miR-155 were evaluated through miRNAs sequencing, transwell assays,western blotting,fluorescence labeling,luciferase reporter assay, and animal experiments.Results: We found that M2 TAMs are abundant in metastatic tissues of NSCLC patients and exosomes secreted by M2 TAMs promote epithelial mesenchymal transition(EMT) and migration of A549 cells.Mechanistically,we demonstrated that miR-155 is the biomolecule in exosomes secreted by M2 TAMs and targets 3’-untranslated regions (UTRs) of RASSF4 to promote NSCLC metastasis.Conclusions: MiR-155 is the key functional molecule in M2 TAMs-released exosomes that promote EMT of NSCLC cells through targeting RASSF4. Our study suggests that miR-155 in TAMs and exosomes may serve as a novel therapeutic target in the treatment of lung cancer.