Metal particle is one of the critical impurities, which has a negative impact on the electric strength of transformer oil. It is essential to detect metal particles in transformer oil to improve the reliability of transformers. However, current detection techniques for metal particle are off-line. The X-ray fluorescence analysis, a promising on-line technique for detection of metal particles has attracted attentions. In this study, the X-ray fluorescence analysis was applied to the detection of metal particles in transformer oil samples, and was compared with the inductively coupled plasma optical emission spectroscopy method (ICP-OES) which is recommended by national standard. Transformer oil samples with various mass fractions (0.1 ~ 100 ppm) millimeter-level iron and copper metal particles were prepared to verify the proposed method. The standard deviation of the results by the X-ray fluorescence exceeds 20% in extremely low concentration (0.05 ppm). And it is also showed that the detection result errors of the X-ray fluorescence and ICP-OES are comparable.