The serial number for your iMac is available in these locations.
On the base of the computer
The serial number with bar codes is etched onto the bottom of the foot of the iMac. The iMac 20-inch and iMac 24-inch models have a printed label with the serial number and Ethernet MAC address.
If you use this method to locate your computer's serial number, hold the sides of the iMac and gently place the computer face down on a soft, clean towel or cloth.
On the original product packaging
If you still have the original packaging for your iMac, you can find the serial number on the barcode label that's affixed to the packaging.
On the original receipt (or invoice)
Some retailers include the serial number of your iMac on the printed or emailed receipt or invoice you received when you originally purchased it.
AppleCare registration numbers and Apple hardware product serial numbers sometimes include the number "0" (zero), but not the letter "O".
Learn how to find the serial number of other Apple hardware products.