Abstract: In this paper we present a detailed characterization of metal-insulator-semiconductor MIS structure with PMMA as insulator layer.The effect of molecular weight on the electrical property of PMMA is researched.The PMMA layer is spin coated from a 20mg/ml dilution of PMMA in chloroform.The critical electric field is more than 1.8MV/cm and the thickness is 220nm.When the molecular is 996K,we get the minimum unit area current,which is 6.0×10-9A/cm².While the molecular is 350K,we get bigger unit area current,which is 8.5×10-9/cm².Schottky emission accounts for the leakage current versus electric field strength behavior in high electric fields and the Schottky barrier height is 0.9eV calculated by linear fitting.The trap density decreases with the molecular weight increasing.996k molecular weight PMMA has the least trap density,which is 4.7×1010/cm².