We report a low-voltage-operated organic field-effect transistor that uses a hybrid gate insulator that has a high dielectric constant k. The gate insulator consists of a high-k polymer cyanoetylated pullulan (CEP) that can be efficiently cross-linked by glycidoxypropyltrimethoxysilane (GOPTMS) at low temperature (~110°C). The very low curing temperature is below the glass transition temperature Tg of conventional plastic substrates for plastic electronics, and is therefore compatible with many plastic substrates for plastic electronics. The cross-linking is very efficient in that only 1/10 (w:w) GOPTMS: CEP produced densely cross-linked thin films with a smooth surface, good insulating property, high capacitance density and high k. The devices functioned at low voltage, and exhibited charge carrier mobility ~ 1.83 cm2 V-1 s-1, and steep substheshold swing ~88 mV dec-1. These results imply that high quality polymer gate insulators are achievable at low temperature with a very small fraction of blended crosslinking agents; this characteristic offers a method to achieve portable all-plastic flexible electronics that function at low voltage.