Invasion　and　metastasis　are　the　basic　characteristics　and　important　markers　of　malignant　tumors,　which　are　also　the　main　cause　of　death　in　cancer　patients.　Epithelial-mesenchymal　transition　(EMT)　is　recognized　as　the　first　step　of　tumor　invasion　and　metastasis.　Many　studies　have　demonstrated　that　cell　fusion　is　a　common　phenomenon　and　plays　a　critical　role　in　cancer　development　and　progression.　At　present,　cancer　stem　cell　fusion　has　been　considered　as　a　new　mechanism　of　cancer　metastasis.　Mesenchymal　stromal/stem　cell　(MSC)　is　a　kind　of　adult　stem　cells　with　high　self-renewal　ability　and　multidifferentiation　potential,　which　is　used　as　a　very　promising　fusogenic　candidate　in　the　tumor　microenvironment　and　has　a　crucial　role　in　cancer　progression.　Many　research　results　have　shown　that　MSCs　are　involved　in　the　regulation　of　tumor　growth　and　metastasis　through　cell　fusion.　However,　the　role　of　cell　fusion　between　MSCs　and　malignant　cells　in　tumor　growth　and　metastasis　is　still　controversial.　Several　studies　have　demonstrated　that　MSCs　can　enhance　malignant　characteristics,　promoting　tumor　growth　and　metastasis　by　fusing　with　malignant　cells,　while　other　conflicting　reports　believe　that　MSCs　can　reduce　tumorigenicity　upon　fusion　with　malignant　cells.　In　this　review,　we　summarize　the　recent　research　on　cell　fusion　events　between　MSCs　and　malignant　cells　in　tumor　growth　and　metastasis.　The　elucidation　of　the　molecular　mechanisms　between　MSC　fusion　and　tumor　metastasis　may　provide　an　effective　strategy　for　tumor　biotherapy.