In　this　research,　we　reported　an　environment　friendly　approach　for　the　synthesis　of　gold　nanoparticles　(Au　NPs)　using　Mentha　Longifolia　leaf　extract.　The　formation　of　the　Au　NPs　was　characterized　by　UV–visible　and　FT-IR　spectroscopy,　XRD,　SEM,　EDX　and　TEM　analyses.　The　UV–Visible　spectra　of　gold　nanoparticles　showed　a　surface　plasmon　resonance　peak　at　512　nm.　The　crystalline　nature　of　gold　nanoparticles　was　stablished　by　XRD　diffraction　pattern.　TEM　revealed　the　spherical　shape　with　a　mean　particle　size　of　36.4　nm.　Thereafter,　biological　performance　of　those　biomolecule　functionalized　Au　NPs　was　investigated.　To　survey　the　anti-human　breast　cancer　effects　of　gold　nanoparticles,　MTT　assay　was　used　on　the　common　breast　cancer　cell　lines　i.e.,　breast　adenocarcinoma　(MCF7),　breast　carcinoma　(Hs　578Bst),　breast　infiltrating　ductal　cell　carcinoma　(Hs　319.T),　and　breast　infiltrating　lobular　carcinoma　(UACC-3133).　The　conversion　was　achieved　in　short　reaction　time　with　good　to　excellent　yields　in　association　with　outstanding　turnover　frequency　(TOF).　In　addition,　the　nanocomposite　catalyst　was　easily　recovered　and　recycled　for　12　successive　times　without　noticeable　decrease　in　catalytic　activity.　Gold　nanoparticles　had　high　anti-breast　cancer　activities　dose-dependently　against　MCF7,　Hs　578Bst,　Hs　319.T,　and　UACC-3133　cell　lines.　The　best　result　of　anti-breast　cancer　effects　was　seen　in　the　case　of　the　UACC-3133　cell　line.　It　looks　gold　nanoparticles　can　be　used　for　the　treatment　of　several　types　of　breast　cancers　in　human.　©　2020