Abstract: Silicon-related defects and vacancy defects caused by irradiation can both cause the blue colour of CVD diamond. Recently, a blue square sample submitted to NGTC labs in Beijing, was identified as a CVD laboratory-grown diamond through magnification observation and luminescence observation under DiamondView^TM, and spectroscopic tests.The infrared spectrum indicated that the sample is type Ⅱa, and the absorption related to hydrogen defects is not detected. The UV-Vis-NIR spectrum showed weak absorption at 830, 856 nm and 946 nm assigned to Si-V⁰, and very strong absorption at 737 nm (Si-V^-)and 741 nm(GR1). A series of absorption generated by GR1 generate a transmission window at the blue area, finally making the sample blue. The photoluminescence spectra revealed obvious characteristics of irradiation, included the defects of strong 503.5 nm(3H), strong 741 nm(GR1), 515.8, 533.6 nm and 580 nm. The defects of H3(503.2 nm) and H2 (986 nm), and the absence of the 596/597 nm indicated that the sample had experienced high-temperature process before irradiation.