We introduce phase-change material Ge2Sb2Te5 (GST) into metal-insulator-metal (MIM) waveguide systems to realize chipscale plasmonic modulators and switches in the telecommunication band. Benefitting from the high contrast of optical properties between amorphous and crystalline GST, the three proposed structures can act as reconfigurable and non-volatile modulators and switches with excellent modulation depth 14 dB and fast response time in nanosecond, meanwhile possessing small footprints, simple frameworks and easy fabrication. This work provides new solutions to design active devices in MIM waveguide systems, and can find potential applications in more compact all-optical circuits for information processing and storage. Keywords: metal-insulator-metal, phase-change material, plasmonic modulator, plasmoninduced transpareny 1. Introduction Facing the increasing demand for information quantity and transmission rate, today's electronic integrated devices will be unable to afford roles in future chipscale systems due to the fundamental limits [1]. As a promising solution, photonic integrated devices can break the blocks by using light as the carrier of information. To construct basic photonic circuits, dielectric waveguides (such as silicon waveguide) have been widely studied for decades [2,3]. However, the scale of a dielectric waveguide cannot go smaller than the working wavelength due to the diffraction limit, which restricts the integration and compactness of photonic integrated chips [1]. Recently, surface plasmon polariton (SPPs), which are the surface electromagnetic waves supported at the interface between metal and dielectric, have attracted more attention due to the capacity to break the diffraction limit [1]. SPPs also introduce plasmonic waveguides, which can deliver light in sub-wavelength scale [4,5]. Especially, plasmonic metal-insulator-metal (MIM) waveguides possess good confinement of light, low bending loss, easy fabrication and acceptable propagation length, therefore can be potential platforms for future more compact all-optical integrated circuits [6][7][8][9]. As the promising next-generation on-chip photonic system, MIM system has supported various chipscale passive devices such as filters [10,11], demultiplexers [12,13], sensors [14,15] and spacers [16,17]. However, active devices are also required in such system. Therefore, optical materials with tunable properties have been utilized to realize dynamically tunable functionalities. For example, nonlinear materials have been used to realize all-optical tunable filters, logic gates and diodes [18][19][20]. Lasers, switches and slow light enhancement can be