As the second son of Trần Nghệ Tông, Ngạc was appointed as Prime Minister with the title Trang Định vương (莊定王, "Prince Trang Định"), but was later killed by an order of Co-Prime Minister Hồ Quý Ly in 1392.
In the next year, he was installed as the new emperor by Đặng Dung and Nguyễn Cảnh Dị in Chi La (modern Đức Thọ District, Hà Tĩnh Province).
Giản Định was arrested by Nguyễn Suý, a general of Trùng Quang, then transferred to Nghệ An Province.
Trùng Quang granted him the title thái thượng hoàng ("Retired Emperor").
Trùng Quang came to Bình Than to fight against the Chinese army, but was defeated by general Zhang Fu and fled back to Nghệ An.