This is from the BootCamp guide.
Removing Windows from Your Computer
How you remove Windows from your computer depends on whether you installed Windows on a single-volume disk or on a second disk partition.
If you installed Windows on a second disk partition: Using Boot Camp Assistant as described below, remove Windows by deleting the Windows partition and restoring the disk to a single-partition Mac OS X volume.
If your computer has multiple disks and you installed Windows on a disk that has only one partition: Start up in Mac OS X and use Disk Utility, in /Applications/Utilities, to erase the disk and reformat it as a Mac OS X volume.
To delete Windows and the Windows partition:
1 Start up in Mac OS X.
2 Quit all open applications and log out any other users on your computer.
3 Open Boot Camp Assistant.
4 Select “Create or remove a Windows partition” and click Continue.
5 Do one of the following:
- If your computer has a single internal disk, click Restore.
- If your computer has multiple internal disks, select the disk with Windows on it, and then select “Restore to a single Mac OS partition” and click Continue.
Thank you very much.
I missed one thing; when I opened Bootcamp assistant I met with two options; a) download content for Windows b) use Mac OSX disk and both of options I met with another problem; your disk is formatted with Mac journaled.
So, I formatted my partition with Windows format and Bootcamp let me do it
Thank you very much for your response.