Some experimental results of in vivo observing X-ray attenuation of intra-tumor injection of indocyanine green (ICG) are reported. An eight-to nine-week-old male BALB/c mouse with mass between 15 g and 20 g is used in the experiments. The mouse is implanted with myeloma cell line (SP2/0) two week before. A system used to monitor the intratumor diffusion of ICG is a digital X-ray imaging system. It works at 33 kVp, 0.3 mA with exposures duration of 4 seconds and magnification of 1.5. The objective of this research is to study the X-ray attenuation at different area, which represented by gray-scale value. Compare to the ROI in the tissue without ICG and ROI of black background in the image, there is an obvious change in the tumor, which is the area ICG can diffuse to. It shows the feasibility of using digital X-ray imaging system dynamically, effectively and noninterventionly to monitor the diffusion of the ICG.