Room-temperature cross-linking of a hyperbranched polycarbosilane (HBPCS) with divinylbenzene (DVB) in the presence of the cyclohexanone peroxide-cobaltous naphthenate (CHP-CN) initiator system was studied. According to the Fourier transform infrared spectroscopy (FT-IR) and H-1 nuclear magnetic resonance (H-1 NMR) results, the cross-linking reaction occurred via the vinyl polymerization. The GPC analysis confirmed the molecular weight of the cross-linked HBPCS significantly increased. Thermal behaviors of cross-linked HBPCS and original HBPCS were investigated by thermal gravimetric analysis-differential thermal analysis (TGA-DTA). The TGA results indicated that the ceramic yield of HBPCS remarkably increased by the cross-linking treatment. For the HBPCS/10 wt% DVB system, the maximum of reaction degree of HBPCS was obtained, which might be responsible for the highest ceramic yield of 70.1 wt% at 1000 A degrees C. However, the ceramic yield of the non-crosslinked HBPCS was only 45 wt% at 1000 A degrees C. The evolution of crystal structure of SiC as a function of pyrolysis temperature was traced by means of X-ray diffraction (XRD) and FT-IR. With the pyrolysis temperature increasing, the beta-SiC peaks became sharper and the grain size also grew larger. As the DVB content increased, the intensity of beta-SiC peaks significantly reduced, indicating smaller beta-SiC grain size.