The fabrication and characterization of a thermal variable optical attenuator based on long-range surface plasmon polariton (LRSPP) waveguide with multimode interference architecture were investigated. The surface morphology and waveguide configuration of Au stripe were studied by atomic force microscopy. The fluctuation of refractive index of poly(methyl-methacrylate-glycidyl-methacrylate) polymer cladding was confirmed to be less than 3×10-4 within 865h curing at 120°C. The end-fire excitation of LRSPP mode guiding at 155065nm along Au stripe indicated that the extinction ratio of attenuator was about 1265dB at a driving power of 69 mW. The measured optical rise time and fall time are 0.57 and 0.8765ms, respectively. These favorable properties promise potentials of this plasmonic device in the application of optical interconnection.