The Nsamya mafic- ultramafic complex of Alaskan type was discovered for the first timein the Ubendian Belt. The study of the origin of mantle- derived magmatism is of great significance to the tectonic evolution history. The Nsamya complex has ring- shape characteristics, clinopyroxene peridotite and gabbro occur from central to northern edge. Zircon U- Pb age is between 1874Ma and 1994Ma, indicating formation in the Late Paleoproterozoic. All the rocks are characterized by low SiO2, high MgO, FeOT , Cr and Ni, enrichment of LREE and LILE (Ba, Pb, U), depletion of HREE and HFSE(Nb, Ta, Zr, Hf and Ti) suggesting that the Nsamya complex is the Alaskan- type formed in anarc setting. Based on the regional geological background, it is suggested that the rocks were formed during the closure of the intra- arc basin inthe late stage of the Ubendian orogeny in the Paleoproterozoic, the magma came from the lithospheric mantle metasomatized by subduction fluid and experienced the crustal contamination of lower crust during underplating.