Layered semiconductors such as transition metal dichalcogenides (TMDs) with proper bandgaps complement the zero-bandgap drawback of graphene, demonstrating great potential for post-silicon complementary metal-oxide-semiconductor technology. Among the TMD family, molybdenum disulfide (MoS2) is highly attractive for its atomically thin body, large bandgap and decent mechanical and chemical stability. However, current nanofabrication techniques hardly satisfy the requirements of short channel and convenient preparation simultaneously. Here, we demonstrate a simple and effective approach to fabricate short channel chemical vapor deposition (CVD) monolayer MoS2 field-effect transistors (FET) with channel length down to 20 nm. Electron-beam lithography based on high-resolution negative-tone hydrogen silsesquioxane electron resists were applied to create 20 nm wide SiO x lines, defining the short channel length. The 20 nm MoS2 FET displays ON-sate current in excess of 100 μA μm-1. The corresponding current ON/OFF ratio at room temperature reaches 105. We carefully studied the short channel effect of as-fabricated MoS2 FETs. Combining with the large-scale growth of CVD method, our results will pave a way for short channel device applications based on atomically thin two-dimensional semiconductors.