Root Level disappear when import fbx

I am facing a problem when importing the .fbx file.

In Maya, the hierarchy of the model is liked below:
Body_Root
|- Bone
|- Mesh
|- Morph

But when I import this .fbx (named Body.fbx) to Unity. The hierarchy becomes:
Body
|- Bone
|- Mesh
|- Morph

I wonder if there is any rules in defining model hierarchy, or how can I import the .fbx file same hierarchy as what I defined in Maya.

Thank you.