Abstract: Alatengsuoyinbo region in Khovd, Mongolia, is located in the southern Altay tectonic zone which possesses a series of arcbasin and accretionary complexes. There is a nearly EW-trending gabbro belt in this region. This paper reports the zircon age of gab-bro and its tectonic environment.15 grains of gabbro zircon are assigned to two categories:one type of zircons has obvious band rhythm and the other type of zircons is uniform and clean without inclusion. In consideration of the high Th/U ratios (0.46~0.68), it is thought that they are typical basic magmatic zircons. 15 zircons yielded a weighted mean ²⁰⁶Pb/²³⁸U age of 317.6±1.6Ma. The gab-bro was formed during Late Carboniferous. The research shows that the gabbro was formed at the end stage of the plate subduction and collision, being the product of mafic magmatic activities under the background of Altay orogeny.