The lattice period gradually changing and its ultra-wide band gap property were studied in the three-dimensional diamond photonic crystals (PC) fabricated by rapid prototyping and gel casting techniques using alumina. The band gap width and center frequency of the band gap almost kept stable when the lattice period changed along the direction in vertical to the propagation direction of the electromagnetic wave. When the lattice period stretched along the propagation direction of the electromagnetic wave, the center frequency of the band gap shifted to the lower frequency range while the band gap width increased slightly. Several PCs that stretched along the electromagnetic wave propagation direction were combined together to investigate their complex band gap properties. The band gap width increased with the period change of the combined PCs. When the period change reached 134%, the band gap width became the widest and was 153% of that of the perfect PC, which agreed well with the simulation results by Ansoft HFSS.