We propose a simple, systematic, and efficient method to enlarge the gap of a photonic crystal. In this method, we add a small fraction of a third component into the existing photonic crystal. The dielectric property of the third component as well as its insertion position in a unit cell are chosen according to the field-energy distribution of two Bloch states at the band edges. The method is very general. It can be applied to any microstructure and is independent of the dimensionality of the original photonic crystal. Thus, it opens up a way to engineer photonic band gaps. Here, we demonstrate the validity of this method explicitly in two dimensions for both s and p waves. We show that, for certain microstructures, absolute gaps can be signifi-cantly enlarged. The method is also demonstrated in microwave experiments.