A study is conducted to present the design and fabrication of topgate HfS2 field-effect transistors (FETs) with high- k HfS2 dielectric. Top-gate HfS2 FETs with ultrathin HfO2 as dielectric are successfully demonstrated, with Al and Y as buffer layer. The on/off ratio of FETs is as high as 105, and the subthreshold swing (SS) is as low as 95 mV dec-1. Top-gate HfS2 FETs with 5 nm HfO2 dielectric can still exhibit excellent properties without any functionalization. Top-gate HfS2 phototransistor also exhibits excellent optoelectronic characteristics with high responsivity of 45 mA W-1. and fast response time of 650 ms.