A type of modulator based on a shallow-etched photonic crystal (PC) slab of silicon-on-insulator material with electro-optic (EO) polymer cladding is designed and investigated. The transmission spectra of the PC slab with the EO polymer are calculated using a finite-difference time-domain method. The band structure and the field distribution of the guided mode resonance are calculated and analyzed. The modulation voltage and bandwidth of the hybrid modulator are simulated. It is shown that flexible designs of low-voltage modulation (0.2 V) or high-bandwidth modulation (62 GHz) can be obtained with the hybrid modulator.