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- Make sure your Mac's volume is turned up enough for you to hear the serial number. 

- Hold the Option key and choose Apple () menu > System Information (or Apple menu > System Profiler in Mac OS X v10.6).

 
 - Press Command-4 (⌘4) or choose File > Speak Serial Number and your Mac will speak its serial number. 

Learn more
You can also find the serial number in the About This Mac window.