Background:　Ganoderma　lucidum　(G.　lucidum)　is　a　traditional　Chinese　herbal　medicine　that　has　shown　potential　as　an　alternative　adjuvant　therapy　for　cancer　patients.　However,　the　mechanisms　and　adjuvant　therapeutic　effects　of　G.　lucidum　in　cancer　treatment　remain　unclear.　Methods:　In　this　work,　G.　lucidum　spore　oil　(GanoOil),　a　newly　developed　oily　G.　lucidum　spore　extract　was　used　to　investigate　the　mechanisms　and　adjuvant　therapeutic　effects　of　GanoOil　in　conjunction　with　the　chemotherapeutic　drug　cyclophosphamide　(CTX)　for　preventing　breast　cancer　metastasis.　Results:　In　the　model　of　lung　metastasis,　orally　administered　GanoOil　increased　the　population　of　CD8+　T　cells　and　interleukin　(IL)-6　cytokine　levels　in　mouse　blood,　whereas　also　enhancing　the　activity　of　natural　killer　cells　in　the　spleen.　Furthermore,　the　combination　of　GanoOil　and　CTX　effectively　suppressed　the　lung　metastasis　of　circulating　breast　cancer　cells,　alleviated　CTX-induced　weight　loss,　and　reduced　the　ratio　of　lung　and　spleen　weight　to　body　weight　in　mice.　Moreover,　high　concentrations　of　GanoOil　exhibited　no　significant　toxicity　or　side　effects　in　both　in　vitro　and　in　vivo　experiments.　Conclusion:　In　conclusion,　GanoOil　is　a　safe　drug　that　can　enhance　immune　activity　in　mice　to　achieve　therapeutic　effects　on　cancer,　and　can　also　synergistically　inhibit　tumor　metastasis　with　CTX.