It runs from Illinois Route 159 north of Bunker Hill to the city of Mount Olive at Illinois Street, a former alignment of U.S. Route 66.

[1] Illinois 138 runs east–west, with a bend north through the city of Benld.

It is a two-lane surface road for its entire length.

South of Benld, it also overlaps Illinois Route 4 for a few miles.

It was put on its current alignment in 1942, replacing parts of Illinois Route 38.