If you are a widow or a widower, you must submit the following:
USCIS Form I-751 (Petition to Remove the Conditions on Residence)
Copy of USCIS Form I-551 (Permanent Resident Card)
Evidence that you did not get married to evade the immigration laws of the United States. Examples of such evidence includes, but is not limited to:
Leases showing that you and your spouse lived in the same place
Documents that prove that you and your spouse owned property together
Birth certificates of your children
Copy of your spouse's death certificate.
http://uscis.gov/graphics/howdoi/remcond2.htm