Here, we demonstrate an ultrasensitive olfactory system, namely, a bioelectronic nose (B-nose), based on CMOS-compatible silicon nanowires (SiNWs) conjugated with an odorant-binding protein (OBP). The SiNWs were low-cost and top-down fabricated with a conventional microfabrication process by using a novel controllable size-reduction process. The OBP-SiNW devices detect vapor analytes under ambient air conditions, with reversible and reproducible sensor responses. The SiNW based B-noses have shown ultrahigh sensitivity for recognizing target odorant as low as 10 ppb and high specificity for discriminating odorant with similar structure.