A THz near-field transmission imaging system was successfully demonstrated to image the vessels inside the ears of nude mice in vivo. Wave-guided illumination and near-field scanning detection with a sub-wavelength aperture were applied. An operating frequency of 340 GHz was chosen to achieve a higher penetration depth in tissues with a reasonable signal-to-noise ratio. The near-field pattern of the power transmittance through the vessel was also numerically simulated and showed good correspondence to the measured results. The capability of the system for long-term monitoring in vivo was also demonstrated.