The route continues east-northeast through more farmland and woodland before passing through rural residential areas and coming to an interchange with I-376, becoming a divided highway at this point.

Past this interchange, the road becomes undivided again and passes more homes before heading through woods and turning southeast, crossing the Mahoning River.

[2][3] At this point, the route turns northeast to join PA 18 on Montgomery Avenue, fully entering New Castle and crossing Norfolk Southern's Youngstown Line.

The road heads east-northeast over the Shenango River and another New Castle Industrial Railroad line, coming to an intersection with PA 168.

A short distance past this intersection, the route heads into Scott Township and passes through McCaslin, running through more areas of farms and woods with occasional residences and turning more to the east-northeast.

The route passes through Elliott Mills, at which point it crosses into Plain Grove Township for a short distance.

[2][3] PA 108 enters Worth Township in Butler County and becomes New Castle Road, turning east-northeast and coming to an interchange with I-79.

Farther northeast, PA 108 heads into the borough of Slippery Rock and becomes New Castle Street, passing through wooded areas of homes.

The route crosses back into Slippery Rock Township and becomes Franklin Road, heading through a mix of farmland and woodland with occasional homes.