Almost 100 firefighters have now brought the blaze in the Sperrin Mountains in Co Derry under control.
15 fire crews worked overnight to prevent the fire in Broughderg Forest close to Six Towns, near Draperstown, from spreading.
The Northern Ireland Fire and Rescue Service said the rugged terrain and lack of water supplies made its task more difficult, while strong winds helped the flames spread through the treetops.
The cause of the blaze, which covered the area of 60 football pitches, has yet to be established.
