The Norwegian Nobel Committee and the UN both urged China to free Liu Xia.
She has been held under house arrest without charge since 2010.
China has rejected international criticism for not allowing Mr Liu, who had been serving an 11-year prison term for “subversion”, to be treated abroad.
The Nobel Committee, which gave him the Peace Prize in 2010, said Beijing bore a “heavy responsibility” for his death.
The committee’s leader, Berit Reiss-Andersen, says the Chinese consulate in Oslo has refused to receive her visa application for travel so she can attend a possible funeral for Mr Liu.
She told the BBC she wanted to show her respect for the dissident’s family “in this tragic situation” but that Beijing had missed an opportunity to make a “fantastic gesture” by letting her do so.BBC