Abstract: A lot of yellow HPHT synthetic diamonds were found in hundreds pieces of cluster set diamond jewelry, which were received by National Gold & Diamond Testing Center. The HPHT synthetic diamonds were tested systematically by using gem microscope, infrared spectrometer,X-ray fluorescence spectrometer, ultraviolet-visible spectrophotometer, ultraviolet fluorescent lamp and DiamondView^TM, The results show that the colour of HPHT synthetic diamonds is relatively consistent yellow with a large number of rod-like, columnar,fine particulate iron-nickel alloy inclusions. Almost all of these synthetic diamonds are magnetic, even strong magnetic. HPHT synthetic diamonds have an apparent infrared absorption peak at 1 131 cm^-1. It indicates these diamonds belong to type Ib diamond which are not common in natural diamonds. Strong peaks of iron and nickel could be tested by X-ray fluorescence. HPHT synthetic diamonds almost exhibit yellow-green fluorescence under shortwave UV fluorescence HPHT synthetic diamonds display yellow-green fluorescence in varying degrees under the DiamondView^TM, and individual shows a black cross phenomenon.