You could use the Seamless Mosaic tool if your images have spatial reference:
http://www.harrisgeospati.../MosaicSeamless.html

There is also a pixel based mosaicking for images without spatial reference.

I tried seamless mosaic before posting the question and it didn't work. I have masked area in my classification and didn't use the mask for mosaic, so got entire out put black. So I gave up that method. After your reply, I tried it again applying the mask and it worked. Just wanted to share what I did wrong, so might help someone.
Thank you so much for your help Mari!

I'm sorry, I have an additional question. Mosaic tool worked for the classified images that had exactly the same classes in same order. But it doesn't work for the tiles with different classes. How do I fix this?

This is a problem when you subset images and classify them separately. It can turn out that some classes exist in some areas and not others, or areas of overlap are classified differently so when you mosaic them back together, the classes don't match.

I am not sure I understand what problem you encountered originally that caused you to break the image up in tiles. I think it is best to use the full image for classification because of this problem.
There is a tool in ENVI called Edit Classification Images that might help here but it is a rather manual process:
https://www.harrisgeospatial.com/docs/EditClassificationImages.html