An automatic mesh generator AUTOMESH-3D is developed by the authors in this paper. The generator can adaptively produce hexahedral element meshes with an improved inside-out grid-based method. This method involves seven steps: STL model construction, geometric feature identification, cubic grid structure generation, jagged core mesh generation, surface-gap filling, boundary matching and quality improvement. Adaptive refinement, local refinement and boundary matching techniques are employed to generate eligible elements according to the model’s features and users’ requirements. Several examples of hexahedral element mesh generation with AUTOMESH-3D are given, and the effectiveness and robustness of the mesh generator are demonstrated.