Solution-processed small molecular semiconductors (soluble acenes) serve as ideal active materials for high-performance, printed organic thin-film transistors (OTFTs). However, device stability of soluble acene TFTs has not been intensively studied. Here, we showed that device stability of soluble acene TFTs can be significantly enhanced by the use of polymer/inorganic hybrid gate-dielectrics. Triethylsilylethynyl-anthradithiophene (TES-ADT) TFTs fabricated on polystyrene (PS)/$Al_2O_3$ hybrid gate dielectrics exhibited high performance such as high field-effect mobility ($0.6cm^2/Vs$) and on-off current ratio ($10^6$). In addition, transfer characteristic of the TFTs did not shift under the prolonged gate-bias (8000s). This outstanding device stability is due to the device structure which consists of highly crystalline TES-ADT crystals and trap-free PS/$Al_2O_3$ dielectric.