George Woodrum, 69, of the 2600 block of Forest Avenue in Dover, Delaware was taken into custody on Thursday in Stafford, Va.
Police say they started investigating Woodrum in October after the 13-year-old girl's mother, who lives in Stafford, called them to say he had been asking the teen inappropriate questions.
That's when a detective assumed the girl's identity during e-mail exchanges with Woodrum.
Investigators say Woodrum began asking for sexually explicit pictures, and asked to have sex with his great-niece.
On Thursday, police say Woodrum drove to the girl's house in Virginia, thinking he was there for sex with her.
Instead, he was arrested.
Police are asking that if anyone has been inappropriately approached by George Woodrum to contact the Stafford County Sheriff's Office at 540-658-4400 or the Delaware State Police at 302-697-2104 Ext. 309.