We have theoretically investigated the tunability of the omnidirectional reflection band (ORB) of a one-dimensional (1D) photonic crystal (PC) consisting of alternating isotropic dielectric and nematic liquid crystal layers by an external electric field. The width of the ORB becomes wide as the external voltage increases, but the center frequency is changed little. The frequency of the defect mode created by inserting a layer of liquid crystals into a 1D dielectric PC is also found to be tunable by the application of external voltage. These properties can be applied to tunable optical filters or optical switches. (C) 2001 American Institute of Physics.