Your certificate of naturalisation number can be found only in one place, your certificate of naturalisation. All naturalised citizens receive certificates upon completing the oath of allegiance to the United States, and it is up to the citizen to maintain that number for safe keeping. The naturalisation certificate number is often necessary to list when applying for petitions or visas for alien family members. If the certificate should become lost, stolen or destroyed, a replacement certificate can be applied for through the U.S. Citizenship and Immigration Services (USCIS).
Refer to your certificate of naturalisation that was awarded to you after the citizenship ceremony. Locate the red number in the top right corner. The number should be preceded by the black abbreviation "No." and should contain eight digits.
Do not confuse the second number printed lower on the certificate that begins with "A" with the certificate number. The second number is your alien registration number, the same number that was on your green card.
Apply for a replacement certificate of naturalisation. If your certificate has been lost, stolen or mutilated, the only way to retrieve the number is to apply for a new certificate of naturalisation through the USCIS. Download Form N-565 from the website of the USCIS and attach two colour passport-style photos with your name and A-Number written on the back of each with pencil. You must also include the application fee of £247, as of 2010, in the form of check or money order made payable to the U.S. Department of Homeland Security before submitting your application to the USCIS. For the list of mailing address, refer to the website of the USCIS.