PA 318 begins at the Ohio border in Shenango Township, where the road continues west into that state as SR 304.

From the state line, the route heads east on two-lane undivided Hubbard-Middlesex Road, passing through areas of farms and woods with some homes.

The route heads into the borough of West Middlesex and becomes Main Street, crossing the Shenango River into residential and commercial areas.

PA 318 continues past more homes prior to heading back into Shenango Township, where it becomes Mercer West Middlesex Road.

PA 318 passes through a mix of fields and forests as it turns northeast onto Pulaski Mercer Road.