Owing to their abundant unique properties and ready compatibility with Si microelectronic technology, Si nanostructures are becoming one of the most important classes of nano semiconductors. Particularly, small-sized Si nanoparticles possess distinctive photoluminescence (PL), biocompatibility, and active surface properties. In this review, we focus on the synthesis and PL properties of small-sized Si nanoparticles and their potential applications in the fields of bioimaging and nanocatalysis. We further highlight major challenges and promises in this area.