Methods of manufacturing stacked nanowires MOS transistors are disclosed. In one aspect, the method includes forming a plurality of fins along a first direction on a substrate. The method also includes forming stack of nanowires constituted of a plurality of nanowires in each of the fins. The method also includes forming a gate stack along a second direction in the stack of nanowires, the gate stack surrounding the stack of nanowires. The method also includes forming source/drain regions at both sides of the gate stack, the nanowires between the respective source/drain regions constituting a channel region. A stack of nanowires may be formed by a plurality of etching back, laterally etching a trench and filling the trench. The laterally etching process includes isotropic dry etching having an internally tangent and lateral etching, and a wet etching which selectively etches along respective crystallographic directions.