A girl introduced a boy as the son of the daughter of father of her uncle. The boy is girl's :

Step 1: Analyze the phrase: "Son of the daughter of father of her uncle."

Step 2: "Her uncle" refers to the girl's uncle, i.e., her father's or mother's brother.

Step 3: "Father of her uncle" refers to the girl's grandfather.

Step 4: "Daughter of father of her uncle" means the daughter of the girl's grandfather — this could be the girl's aunt (father's sister).

Step 5: "Son of the daughter of father of her uncle" means the son of the girl's aunt.

Step 6: The son of the girl's aunt is her Cousin.

Conclusion: Therefore, the boy is the girl's Cousin.