The morphology of an irregular particle plays a key role in its interactions with the surroundings, and the 3D characterization of an irregular particle is of essential importance. Digital inline holography in Gabor configuration is applied to measure the 2D shadow texture and 3D position, as well as the 3D boundary line of irregular particles. The depth position of the 3D boundary line can be determined with a criterion of the local deviation of the directional gradient over a small tilted rectangle, which covers the interrogated boundary section. The method is firstly verified by rigorous simulated particle holograms and then by accurate experimental fiber hologram. The 3D boundary line of an irregular coal particle is also measured. Results show that the 3D boundary line of an irregular particle can be accurately retrieved from the reconstructed volumetric optical particle field. This capability of 3D boundary line measurement enables the application of digital inline holography to the 3D diagnostic of the morphology and dynamics of irregular objects, including 3D rotation. (C) 2016 Elsevier B.V. All rights reserved.