It runs mostly west-east, on mostly existing township and range roads in the Municipal District of Greenview No.

The highway starts on Range Road 85 at a creek crossing, south of this point Weyerhaeuser Canada Corporation maintains a network of forestry roads, extending approximately 100 km (62 mi) south to the Two Lakes Provincial Park and Nose Mountain camping areas, and further to the 30 m (98 ft) high Kakwa Falls on the Kakwa River.

It then follows the south bank of Wapiti River, with O'Brien Provincial Park in between, before meeting up with Highway 40.

[1] The Highway 40 bridge over Wapiti River south of Grande Prairie also crosses Highway 666, and with separate northbound and southbound exits to Hwy 666, it makes a Trumpet/Directional-T hybrid.

[3] It is used by Forestry and the Oil and Gas industry working in the area of the Wapiti River valley, as well as for recreational use (two summer campgrounds are open in this wilderness area).