When I take http://tes.nexusmods.com/downloads/file.php?id=23487 out of my inventory, it doesn't drop to the floor but stay in the air instead. It seems the objects lack physics effect or something, and I wonder how to fix it. Could someone tell me about it? Thanks a lot.

Sounds like an improper _gnd.nif. "Ground" nifs should be statics.

Could you tell me how to correct it? What tool should I use?
Thanks again.

You need to use Blender. Its...not hard, but I don't have time to go over it at the moment.

As Nephenee13 said, use Blender if you want to create from scratch.

But, you can also 'quick and dirty' fix using existing meshes with the Construction Set. Open with 'Oblivion.esm' checked (presuming it is the master of the mod, like most) and the mod in question checked and set as the 'active plugin'.

Then, in the 'Object Window', locate the item in question by it's 'Editor ID' (TES4Edit/View is good for finding that), and double-click on the item. That will open the 'Edit Window'.

In the right-hand half you will see 'Male' and 'Female' columns. The second entry of each is the 'World Model'. That is where you point to the '*_gnd.nif' mesh you want to use. (Just look at a similar existing item in the same way.) That mesh has the Havok physics applied and is what the 'dropped' item looks like when lying on the ground, and as such can be the same '_gnd.nif' as any other similar item. If you want to change anything about it such as the color, make a copy and name it so the first part of the filename matches the 'nif' filename in the 'Biped Model' but be sure to retain the '_gnd.nif' ending.

If it's missing the '_gnd.nif' mesh, it may also be missing the inventory icon which is pointed to in the 'Icon Image' field. Those are 'dds' files located under the 'Textures\menus\icons' folder. Any icon used for similar items will do; most modders don't create new ones but just want to have it indicate the general category of item type.

If you use the 'Edit Asset Path' button in the 'Asset Selector' displayed when you click on the 'Add NIF' or 'Add Icon' buttons, you can copy and paste the desired path and filename between items. Just be sure your path is 'relative', meaning it must start below 'meshes' or 'Textures' and not with drive letters.

(Wow. Someone asked about practically the only thing I've managed to do in a mod so far.)

-Dubious-

I pulled the file and looked at it. It does have it's own custom gnd nifs, but a quick look at them in nifskope shows incorrect collision type, hence the odd behaviour.

You will need Blender to fix this. (Blender 2.49b is best for Oblivion)
Here is a good tutorial on http://tes.nexusmods.com/downloads/file.php?id=19739 for armour gnd meshes. When you import the _gnd.nifs into blender make sure to delete the existing collision first.