how to remove background of placed object in illustrator cs5

Using the magic wand or lasso tools or a combination, select the background as well as you can, then select inverse. Now the object will be selected.

In Window>Path choose "Make Work Path" from the flyout menu on its upper right corner. Type in .5 for the tolerance and hit okay.

Now in the Path Window double click on the new path you made "Work Path" and the Save Path dialog appears. Name it "Path 1" (it already should say this in the dialog).

Then, using the Path Window flyout menu again, select Clipping Path and choose "Path 1." Leave flatness blank.

Now save the document as a PhotoShop EPS and place into Illustrator.

The path you made is now the clipping mask in Illustrator. You can edit the clipping mask's path's anchor points to refine it even more using the white arrow tool... if there's background still peeking out you can move the path closer to the object where need be.

That's still my favorite way. The magic wand tool in PhotoShop can usually get pretty close to the object.

But if you've got a version of Illustrator that features Live Trace, and the object is pretty well differentiated from its background (like a dark object on a solid light background), you can use Live Trace to create a path around the object, then use that path as a clipping mask.
1 - Place the image (not linked)
2 - Select Object > Live Trace > Tracing options and set the settings to the preset. Try 'Simple Trace'. You can use the Preview feature to see what you're going to get.
3 - Hit Trace.
4 - Now select the new object and select Object > Expand.
5 - Now ungroup.
One of the paths that is created when you expand will probably be a perfect outline of the object*. Finding that one path may be difficult. Use the black arrow tool to delete paths until you find the one that is essentially the outline of the object. Get rid of all the other objects and copy that "perfect" path.
6 - Place the image again. Paste the perfect path on top of the image and use the black arrow tool to position it where you want it. Choose both the image and the path and hit the Ctrl+7 keys to create a mask.

*This method works really well with simple, well-defined images but it's harder to get it to work with complicated images with messy backgrounds. I still use the first (PhotoShop) method when I'm creating works for print.

Hope this helps you !

change backeground color ??

from white to some other colors?

sorry,i didn't find that,seems no where to change it~