All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah and that Muhammad is His slave and Messenger.
It is permissible for the Muslim man to marry the daughter of his father's paternal uncle; she is among the category of women to whom marriage is lawful in Islam as she is held in the same legal status of a cousin, the daughter of one's paternal uncle in this regard.
Allaah, the Exalted, says (what means): {...and the daughters of your paternal uncles and the daughters of your paternal aunts and the daughters of your maternal uncles and the daughters of your maternal aunts...} [Quran 33:50]
Ibn Katheer wrote:
"...This is justice which avoids going to either extreme; the Christians do not marry a woman unless there are seven grandfathers between the man and the woman (i.e., they are very distantly related or not at all), and the Jews allow a man to marry his sister's daughter. So, the pure and perfect Islamic sharee'ah came to cancel out the extremes of the Christians and permitted marriage to the daughter of one's paternal uncle or aunt or the daughter of a maternal uncle or aunt and forbade the excesses of the Jews, who allowed marriage to the daughter of a brother or sister, which is an abhorrent thing." [Tafseer Ibn Katheer]
For more benefit, please refer to fatwa 85868.
Allaah knows best.