Side-polished fiber immunosensor based on surface plasmon resonance (SPR) onto self-assembled protein A layer was proposed for the detection of <i>Legionella pneumophila</i>. A self-assembled protein A layer on gold (Au) surface was fabricated by adsorbing a mixture of 11-mercaptoundecanoic acid (MUA) and activated by N-Ethyl-N'-(3-dimethylaminopropyl) carbodiimide/ N-Hydroxysuccinimide (EDC/NHS). The formation of self-assembled protein A and gold layer on side-polished surface and the binding of antibody and antigen in series were confirmed by SPR response on spectrum. The binding protein A layer can improve the sensitivity, which indirectly supports the configurations that antibody layer is immobilized on the binding protein A layer with a well-ordered orientation. The surface morphology analyses of self-assembled protein A layer on Au substrate and monoclonal antibody against <i>L. pneumophila</i> immobilized on protein A were demonstrated by SPR dip shifts on optical spectrum analyzer. The SPR fiber immunosensor for detection of <i>L. pneumophila</i> was developed and the detection limit was 10 CFU/ml with the SPR dip shift in wavelength from 1070 to 1105nm. The current fabrication technique of a SPR immunosensor using optical fiber for the detection of <i>Legionella pneumophila</i> could be applied to construct other biosensor.