2D semiconductors are considered to be one of the most promising channel materials to extend transistor scaling. However, the integration of ultra-thin dielectrics on 2D semiconductors has been challenging, and the reliability has not been investigated to date. Here, using monolayer 3, 4, 9, 10-perylenetetracarboxylic dianhydride (PTCDA) molecules as interface layer, we realize EOT as low as 1.7 nm on large-area monolayer CVD MoS2. The reliability of ultrathin high-κ dielectric on 2D semiconductors is systematically studied for the first time. The median breakdown (BD) field of HfO2/PTCDA stack is over 8.42 MV/cm, which is two times that of HfO2/Si under the same EOT. Through TDDB we project that the gate dielectric can work reliably for 10 years under EBD = 6.5 MV/cm, which shows 85% improvement than HfO2/Si. The BD current increase rate in our gate stack is several orders of magnitude smaller than HfO2/Si. The excellent reliability suggests that molecular interfacial layer is a promising dielectric technology for 2D electronics.