Ultralow-loss thermosetting resins cured via free-radical polymerization have been extensively applied as the polymeric matrix in high-frequency and high-speed printed circuit boards and electronic packaging substrates. In recent years, silicon doping has been commonly acknowledged for its potential to reduce the dielectric loss of materials. Three silicon-containing cross-linkers are investigated in this work for use in ultralow-loss thermosetting poly(phenylene oxide) (PPO) materials. The study compares the influence of these silicon-containing cross-linkers with two commercially available cross-linkers, focusing on their effects on curing temperature, dielectric properties, thermal characteristics, moisture resistance, and aging resistance. Silicon-containing cross-linkers showed appropriate reaction temperatures that were comparable to that of conventional epoxy materials. More importantly, they decreased the dielectric loss of cured samples with the lowest dissipation factor (D f ) value of 0.00159 at 10 GHz. The addition of silicon atoms also slowed the deterioration of dielectric properties during high-temperature aging experiments and reduced moisture absorption in PPO samples. However, there are also concerns regarding the reduction in the glass-transition temperature and the increase in the coefficient of thermal expansion. These results demonstrate the promising potential of silicon-containing cross-linkers in enhancing the performance of ultralow-loss PPO materials for advanced electronic packaging applications.