Hi Lara,

re:
> I only want to understand why the filleting failed at sweptAtOnce version..

Yes like Phiro writes above it's the same answer from earlier in the thread here:
http://moi3d.com/forum/index.php?webtag=MOI&msg=10096.34

Filleting is only possible at edges that are sharp, not at edges that are smooth (tangent) where surfaces meet up.

Additionally, you would need to have an edge between 2 different surfaces to make an edge fillet, if you delete your curves used to generate the sweep you should see that your sweep generated a single large smooth surface.

re:
> It is a sweep with only circles. Do not know why the generated geometry has soft edges.

Because that's how sweep works, if the path for the sweep is smooth you get a smooth surface generated from it. Sweep will only make a sharp cornered shape in places where the path curve also has sharp corners in it.

- Michael