Dear Mr. Chung,
Thank you for posting your request on History Hub!
Naturalization records dated prior to October 1991 from the Federal courts are at the National Archives. If you know the city and state where your father’s naturalization occurred, you can reach out to the NARA office that maintains records of the Federal court in that area. If a naturalization took place in a Federal court, naturalization indexes, declarations of intention (with any accompanying certificates of arrival), and petitions for naturalization will usually be in the National Archives facility serving the state in which the Federal court is located. No central index exists. To ensure a successful request with the National Archives, researchers should include: the name of petitioner (including known variants); date of birth; approximate date of entry to the US; approximate date of naturalization; where the individual was residing at the time of naturalization (city/county/state); and country of origin.
In most cases, the National Archives will not have a copy of the certificate of citizenship. Two copies of the certificate were created – one given to the petitioner as proof of citizenship, and one forwarded to the Immigration and Naturalization Service (INS).
All INS records are now overseen by the US Citizenship and Immigration Services (USCIS). USCIS maintains duplicate copies of court records (including the certificate of citizenship) created from September 27, 1906 through March 31, 1956 within Certificate Files (C-Files). Beginning on April 1, 1956, INS began filing all naturalization records in a subject’s Alien File (A-File). C-Files and certain A-Files may be requested through the USCIS Genealogy Program.
Certificates of citizenship were issued by the Federal courts until October 1991 when INS took over responsibility for naturalization proceedings. Certificates may be requested through the USCIS Genealogy Program.
We hope this information is helpful. Best of luck with your family research!