A novel multi-functional linear siloxane compound, a siloxane polymer prepared from the siloxane compound, and a process for forming a dielectric film by using the siloxane polymer. The linear siloxane polymer has enhanced mechanical properties (e.g., modulus), superior thermal stability, a low carbon content and a low hygroscopicity and is prepared by the homopolymerization of the linear siloxane compound or the copolymerization of the linear siloxane compound with another monomer. A dielectric film can be produced by heat-curing a coating solution containing the siloxane polymer which is highly reactive. The siloxane polymer prepared from the siloxane compound not only has satisfactory mechanical properties, thermal stability and crack resistance, but also exhibits a low hygroscopicity and excellent compatibility with pore-forming materials, which leads to a low dielectric constant. Furthermore, the siloxane polymer retains a relatively low carbon content but a high SiO.sub.2 content, resulting in its improved applicability to semiconductor devices. Therefore, the siloxane polymer is advantageously used as a material for dielectric films of semiconductor devices.