The localized surface plasmon resonance (LSPR)-based optical biosensor was used as a potential tool for label-free detection of immunoreaction. The glass substrate covered with the self-assembled monolayer (SAM) of gold colloids was used widely in the sensors. Here, the glass substrate was modified by chemical hydroxylation first, and then gold colloids were immobilized on the substrate by electrostatic adsorption. The LSPR spectra were obtained on UV-vis absorption spectrometer. The specificity was examined by extensive nonspecific binding tests. The resonance condition on the local dielectric environment enables a simple form of molecular sensing. The binding of analyte to the biosensor surface causes a change in the absorbance which was responsive to the concentration of human IgG. So, the LSPR sensing yields similar results to the SPR technique, yet with much simpler instrument.