A one-dimensional Si/SiO2 photonic crystal with a large, tunable air defect cavity is fabricated. Multiple resonant modes are observed within the photonic band gap. The free spectral range (FSR) is large compared to other resonant structures, with more than 100nm bandwidth. Simultaneous low voltage tuning around two telecom wavelengths, 1.55μm and 1.3μm, is realized using electrostatic force. The whole process is at low temperature and can be CMOS compatible. Potential applications include switching, modulation, and wavelength conversion devices, particularly WDM devices.