The authors report on the dielectric and leakage current properties of room temperature grown MgO-Ba(0.6)Sr(0.4)TiO(3) (MgO-BST) composite thin films to be utilized InGaZnO(4) thin films transistors (TFTs) fabricated on a polyethylene terephthalate (PET) substrate. The InGaZnO(4) TFTs with MgO-BST gate dielectrics exhibited low operation voltage of 4 V, high on/off current ratio of 4.13x10(6), and high field effect mobility of 10.86 cm(2)/V s. These results verify that a room temperature grown MgO-BST gate dielectric is a good candidate for producing high performance InGaZnO(4) TFTs on plastic substrates. (C) 2008 American Institute of Physics.