Abstract: Poly(silphenylene-dimethyl siloxane)s (PTMPS-DMS) containing crosslinkable vinyl groups were synthesized through the Si—H/Si—OR polycondensation process. The crosslinked PTMPS-DMS networks were prepared by hydrosilation and the BPO thermal crosslink process respectively. TG，DSC and isothermal gravimetric analysis at 300℃ showed that the crosslink process had great impact on the thermal properties of the crosslinked networks. The BPO thermal crosslinked network，which began to degrade at 480℃ in air，possessed much higher thermo-oxidation stability than that crosslinked by the hydrosilation process，which began to degrade at 368℃ in air. The Tgs of these two networks had no significant difference，which were －55℃ and －49℃，respectively. It showed that the catalyst H2PtCl6 greatly decreased the thermo-oxidation stability of the PTMPS-DMS networks.