Self-assembly (SAM) gold nano-islands are fabricated by two-step thin-film deposition-annealing method. Despite random distribution of the SAM, the p-polarized light after total internal reflection shows significant phase transition at the extinction wavelengths upon refractive index variation due to localized surface plasmon resonance (LSPR) effect. It resembles the sharp phase transition observed in conventional surface plasmon resonance (SPR) biosensors, so that the bulk sensitivity of the SAM-LSPR sensor is improved via the phase interrogation method. In this Letter, we present both computational and experimental investigations to the SAM-LSPR sensor and the results show excellent agreement with each other. With bulk refractive index resolution to 9.75×10(-8) RIU, we believe the phase-detecting SAM-LSPR sensor would be an essential step toward low-cost label-free sensing applications.