Two-dimensional MoS 2 attracts much attention because of its potential application in electronic and optoelectronic devices at present. However, the preparation of large and uniform single crystal MoS 2 nanosheets is still a challenge, which restricts its further application. Herein, monolayer single crystal MoS 2 nanosheets with large and uniform grain size have been synthesized by chemical vapor deposition using a double-tube system. Within the main growth area, the grain size reaches 146 μ m on the substrate of SiO 2 /Si, and the portion of MoS 2 nanosheets with grain size between 50 μ m and 100 μ m is up to 78%. Meanwhile, the grain size keeps constant in the direction perpendicular to the tubes and changes slightly in the parallel direction. This is attributed to the concentration distribution of intermediate product MoO 3−x in the one-side sealed inner tube set in a quartz tube, i.e., the double-tube system, which provides a way to the controllable and uniform synthesis of large monolayer single crystal MoS 2 nanosheets.