Molybdenum disulfide (MoS2), as a representative of two-dimensional layered materials, has been extensively investigated due to their unique structure and interesting electronic and optical properties. However, the controllable synthesis of monolayer and pyramidal MoS2 nanostructures needs improvement, and their growth mechanism requires more investigations. Here, uniform MoS2 nanostructures from monolayer flakes, few-layer pyramids to multilayer blocks were successfully fabricated by a catalyst-assisted thermal-evaporation-based chemical vapor deposition method via simply adjusting the carrier Ar gas flow rate. After the comprehensive characterization on the obtained materials, their nucleation and growth mechanisms were proposed with specifically highlighting the influence of the carrier gas flow rate, which might also be of help to understand the synthesis processes of other two-dimensional semiconducting transition metal dichalcogenides by similar method.