When writing a letter to a nun, use proper etiquette. Formally address her on the exterior of your envelope, and address her properly in the salutation and body of your letter. If you seek a nun's mailing address, you may successfully find it online, provided you know some information such as her name, parish or diocese.
Find the nun to whom you wish to write a letter. If you remember the community or diocese she worked in, you may be able to track down her mailing address online at the CatholicOnline.com.
Address the envelope. If addressing a Mary Smith, the outside of the envelope should read "Sister Mary Smith, OSH." If you are also ordained, you may address the envelope with "The Reverend Mary Smith, OSH."
Write your salutation. A proper salutation for a nun is simply "Dear Sister."
Write the body of your letter politely and professionally. When addressing her directly, use "Sister."
Close your letter and thank her for reading.