A magnetic resonance imaging technique (MRI) for ultra low-temperature physics was developed. The MRI was applied to visualize the shape of a phase-separated boundary of liquid He-3-He-4 mixtures as a function of temperature near the tricritical point. We determined the capillary length and the contact angle of the boundary against a sample cell wall. The interfacial tension calculated from the capillary length agreed with reported values and we estimated the spatial resolution of the MRT to be a few 10 mu m. (C) 2000 Elsevier Science B.V. All rights reserved.