The gradually varied air spheres radii and its controllable bandgap properties were studied in the three-dimensional inverse diamond structure ceramic photonic crystals (PCs). The PCs were fabricated by stereolithography and gel-casting process using alumina slurry. It was found that, as the radius increased, the bandgap width also increased and the center frequency of the bandgap shifted to the higher frequency range. These results agree well with the simulation results by Finite Integration Technique. Several PCs with gradually varied radii were combined together along the direction < 100 > to investigate their complex bandgap properties. Compared with the perfect PC, the bandgap width of the combined PCs increased remarkably with decrease ratio of radius increasing when the electromagnetic wave transmits along the direction in parallel to the combined direction of PCs. When the radius decreased to 20% of the perfect PC, the bandgap width reached 234% of that of the perfect PC. The resulting gap-midgap ratio is 46.2%. When the electromagnetic wave transmits along the direction in vertical to the combined direction of PCs, the bandgap width increased gradually, and that reached the maximum value when the decrease ratio was 20%.