Three-dimensional (3D) photonic crystals with a diamond structure made from 40 vol% TiO2-acrylate dielectric composites were formed by means of a CAD/CAM micro-stereolithography system. The lattice constant of the diamond unit cell was 500 mu m and the forming accuracy was 10 mu m. The photonic band gap in the Gamma-X < 100 > direction measured by terahertz-time-domain spectroscopy appeared at 280-360 GHz, which agreed fairly well with the band gap calculated by the plane wave expansion method.