This L-shaped two-lane highway was first designated in the early 1930s along the stretch between Preble County Road 335 (CR 335) and SR 121.

The north–south portion of SR 320 between New Westville and New Paris runs in parallel to the east of the Indiana state line by a distance of just more than one mile (1.6 km).

SR 320 travels to the west-northwest through rural farmland prior to bending to the west into the hamlet of New Westville, where the highway encounters a primarily residential area.

A combination of woods and homes appear along the west side of the highway at this point, with more farmland abutting it to the east.

After a short reverse curve that takes the highway to the northeast and then back to the north, SR 320 encounters forested land on both sides.