Hyperdoping silicon nanocrystals (Si NCs) to a concentration exceeding the solubility limit of a dopant may enable their novel applications. Here, the successful hyperdoping of Si NCs with boron (B) and phosphorus (P) is demonstrated, which are the most important dopants for Si. Despite the hyperdoping, the diamond structure of Si NCs is hardly modified. There are both electrically active B and P in hyperdoped Si NCs. It is proposed that the hyperdoping is made possible mainly by the kinetics in the nonthermal plasma synthesis of Si NCs. Collision between Si NCs and B or P atoms and the binding energy of B or P at the NC surface are critical to the understanding on the differences in the doping efficiency and dopant distribution between B and P. B-hyperdoping-induced tensile stress needs to be taken into account in the investigation on the doping and oxidation of Si NCs.