This paper reports the effect of applying an Au island film to the hetero-core structured optical fiber, which is fabricated by annealing thin Au films with thicknesses of 3 and 5 nm and its sensing performance for the refractive index changes. We experimentally observe that novel localized surface plasmon resonance spectra for 1.333 RIU appear in the visible-to-near-infrared region, depending on the shape of Au islands. The absorbance and spectral sensitivity for a given refractive index region of the tested solvents were obtained to be 4.81 AU/RIU and 517 nm/RIU, respectively, in the range of 1.333-1.384 RIU in the case of 5 nm thick Au film annealed at 900°C.