A preliminary experiment of X-ray phase microtomography by a single phase grating is reported. A phase grating was placed behind an object and illuminated by spatially coherent X-rays. At a specific distance from the grating, a periodic intensity pattern caused by the fractional Talbot effect was recorded with a high spatial-resolution image detector. A differential phase map related to the object was retrieved from the deformation in the periodic intensity pattern on the basis of the fringe scanning method. Phase tomograms of a piece of polymer blend were reconstructed and a phase-separation structure in the blend was successfully resolved.