SiO2(activated or mesoporous silica)/Mg(magnesiothermic or metal sintering aid)/C(activated or polymeric carbon)/N2(atmosphere) systems were used in the one‐step synthesis of β‐SiC and β‐Si3N4 whiskers. In this study, a mixture of the active precursors was allowed to react via a self‐sustaining reaction (high‐energy ball milling process). Scanning electron micrographs and X‐ray diffraction (XRD) analysis showed that the rod‐like SiC whiskers (~800 µm) were synthesized in situ by the direct carbothermal reduction of silicon nitride (or silicon) with activated carbon in N2 (or Ar) atmosphere. The results show that β‐Si3N4 (without β‐SiC) was fully formed after 5 h of milling with four different morphologies, namely whisker tip (droplet/no droplet) and nonuniform whiskers (short hexagonal/rhombohedral/rod‐like) with a length of 0.1–400 µm. By adding metal sintering aids, the liquid phase Mg–Si–O–N and the rate of carbothermal reduction increased (enhanced densification via particle rearrangement) and their hexagonal whiskers tended to assume a rod‐like shape. The effect of the concentration of CO (reduction of α‐Fe2O3 to Fe by CO) on the whisker synthesis suggests that, in addition to the concentration of CO, the nature of the family of mesoporous silica/carbon template is an important factor in the synthesis of β‐SiC and β‐Si3N4 whiskers. The possible chemical reactions were investigated by studying the unwanted phases (MgO, Si, SiC, Fe2O3, Fe3O4, FeO, Fe, Fe3C, MgCO3) of comparable XRD graphs.