Its western terminus is at Illinois Route 100 in Kampsville, and its eastern terminus is at Interstate 55 in Zanesville Township near Raymond.

This is a distance of 54.06 miles (87.00 km).

[1] Illinois 108 is a two-lane undivided surface road for its entire length.

In Kampsville, the Kampsville ferry operates across the Illinois River and connects the Kampsville segment of Illinois 108 with its remainder.

It is one of two permanent ferries still operated by the Illinois Department of Transportation; the other is located in Grafton near the intersection of Illinois 100 and Illinois Route 3.