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A resolution-tunable localized surface plasmon resonance (LSPR) biosensor employing electro-optically modulated phase interrogation is presented. This biosensor modulates the analyte-dependent LSPR phase characteristic through electro-optic effect by varying the wave vector of lightwave for exciting surface plasmon. The induced LSPR phase change is measured by the collinear heterodyne technique and its relation with the applied voltage is utilized to determine the analyte concentration. Experimental results show that the regression slope of the phase-voltage relation decreases with the analyte concentration and the detection sensitivity can be increased by widening the waveguide width and using thinner gold film beneath gold nanoparticles. Detection resolution of this LSPR biosensor can be enhanced by increasing the applied voltage to enlarge the induced phase change. The presented LSPR biosensor employing phase interrogation has the features of resolution tunability, fast modulation speed, high modulation stability, and noise reduction.