To avoid the effect of the error of dielectric volume fraction brought by fabricating process on band gap, the diamond crystals with different the error of dielectric volume fraction were designed and fabricated to investigate the fluctuation of band gaps. Theses photonic crystals with a diamond structure were fabricated using alumina by stereolithography, gel-casting and sintering. The photonic band gaps were observed along direction and the photonic band gaps were formed in different frequency range. Increasing the radius from 4.2mm to 4.32mm and changing band width from 0.8 to 1.39GHz, the radius is increased by 2.86%, and corresponding band width increased by 73.75%. Therefore, the fabrication error in mold and fabrication process including injecting mold, sintering should be maintained under the same conditions, which can retain the error of band gap.