New composite binder systems have been developed to improve strength of shell mold and to decrease processing time, as functions of dipping time and binder composition, which is compared with the conventional binder system. The new binder systems were prepared by mixing tetraethyl orthosilicate (TEOS) and polydimethyl siloxane (PDMS) as precursor of SiO2 and sodium methoxide (NaOMe) as precursor of NaO2. The samples of shell mold were dipped into the prepared binder systems, and then heat-treated at 1000°C for 1 hr. The fracture strength is sharply enhanced in the new binder system owing to the increase of penetration depth with the dipping time, whereas the conventional binder system is modestly increased even in the dipping time of 2 hr. The new binder system with 7.6 wt% PDMS shows the highest nominal strength, showing a nominal value of 16 GPa. When only PDMS as precursor of SiO2 was used in the new binder system, the increase of viscosity by PDMS causes a larger scattering in the strength value. The strength could be controlled with the dipping time and molecular weight, showing the lower nominal strength values at the dipping time of 0.5 hr and in the low molecular weight of TEOS/NaOMe. The relationship between strength and binder composition is discussed, based on the microstructures before and after heat-treatment.