In this paper, electrical characterization of low-k dielectric methyl-silsesquiazane (MSZ) is presented. Thermal stress and bias temperature stress (BTS) were utilized to evaluate the impact of Cu penetration on dielectric properties. In the investigation of thermal stress performed by furnace annealing, the leakage mechanism of Al- and Cu-gate MIS capacitors is competed by the decrease of the interfacial states between metal and dielectric and the increase of defects resulted from Cu penetration. Also, the leakage conduction mechanism at high electric field is deduced from Schottky emission in conjunction with space-charge-limited current conduction (SCLC) through BTS methods.