Abstract: The defect size in low density powder metallurgical products can be detected by the technology of X ray real time imaging, however, the calculation process is more complicated. In this paper, the ruler for evaluating defect size was designed and investigated. The correlation of ruler position and detection accuracy was analyzed. The process of observing the powder product defect by the technology of X ray real time imaging was simplified, which would improve work efficiency and have important application value in engineering field.