K.B.S.K.B. Jayawardana, K A I L Wijewardena Gamalath, Study on the Photonic Band Gaps of the Face Centered Cubic Crystals, ILCPA Volume 70, International Letters of Chemistry, Physics and Astronomy (Volume 70) https://www.scipress.com/ILCPA.70.63 Abstract: Since the dielectric contrast of photonic crystals play an important role in determining the existence of a photonic gap, the photonic energy bands, density of states of face centered cubic structured photonic crystals formed from spheres of several dielectric materials placed in air were calculated using the plane wave expansion method. A complete band gap was obtained between second and third bands with a gap to mid gap frequency ratio in the range for the dielectric contrast in the range 11-16 with dielectric spheres of radius with a filling factor of 0.134 and for<sub> </sub>dielectric contrast of 200 with . A complete gap was not found for the dielectric contrast of 3.9. A complete band gap can be obtained for filling factors for the dielectric contrast in the range with an optimum band gap for the filling factor 0.134 while GaAs () has almost a constant optimum band gap in this range. The largest gap to mid gap ratio of was obtained for GaP (). For dielectric spheres of and larger gap to mid gap ratio were obtained for the dielectric contrast while the largest were obtained for . The only dielectric material BaSrTiO3 () which gives a band gap for the filling factor of 0.4524 can be used in microwave applications. Keywords: Band Gap, Dielectric Contrast, Face Centered Cubic, Filling Factor, Gap to Mid Gap Ratio, Mode Field Distributions, Photonic Crystal, Plane Wave Expansion Method