The performance limits of monolayer transition metal dichalcogenide ( MX <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ) transistors are examined with a ballistic MOSFET model. Using an ab initio theory, we calculate the band structures of 2-D transition MX <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> . We find the lattice structures of monolayer MX <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> remain the same as the bulk MX <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> . Within the ballistic regime, the performances of monolayer MX <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> transistors are better compared with those of the silicon transistors if a thin high-κ gate insulator is used. This makes monolayer MX <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> promising 2-D materials for future nanoelectronic device applications.