Alabama State Route 127 becomes SR 166 at the Alabama state line in the Bethel community of southern Giles County.

SR 166 goes north as a 2-lane highway and intersects SR 273, and then goes north through rural and hilly terrain before coming to an intersection with US 64 and entering Pulaski.

They then leave downtown and comes to another intersection with US 64, where they become concurrent with US 64 and SR 15 becomes unsigned.

It then continues north to cross into Maury County.

SR 166 then turns northwest to leave Mount Pleasant and pass through rural areas before entering Hampshire and coming to an end at an intersection with US 412 (SR 99).