We report our investigations on the development of a novel self-assembled monolayer (SAM)-based flow-through surface-plasmon-resonance (SPR) immunosensor for highly specific and rapid detection of benzaldehyde (BZ). The functional sensing surface was immobilized with a BZ derivative on the SAM of polyethyleneglycol-alkanethiols. Both the selfassembling of alkanethiols and the immobilization of BZ were controlled by the analysis of SPR resonance angle measurements. The BZ-bound SAM surface shows high specificity for immunoaffinity reaction with anti-BZ antibody (BZ-Ab). The quantification of benzaldehyde is based on the principle of indirect competitive immunoassay, in which the immunoreaction between the BZ derivative on sensor surface and anti-BZ antibody was inhibited in the presence of free BZ in solution. The decrease in SPR angle is proportional to the increase in the concentration of BZ in test samples. The SPR immunosensor exhibited excellent sensitivity for the detection of BZ in the concentration range from 0.1-80 ppb (ng ml-1). Detection limit of the present sensor is superior compared to various high-performance SPR and QCM sensors. Similarly, HPLC and GC techniques reported a low-detection-limit of only 10 ppb BZ, despite a set of sample pretreatments. In addition, enhancement of the SPR sensor signal and the sensitivity were investigated by introducing a single-step add-on strategy involving either Au nanoparticles or a simple biotin-avidin coupling reaction, with which the detection limit has been improved dramatically.