Regionally averaged heat transfer distributions and friction factors in square channels with twisted tape inserts and with twisted tape inserts plus interrupted ribs are experimentally investigated. The effects of surface heating condition on heat transfer enhancement are also investigated. Each wall of the square channel is composed of isolated aluminum plates. The interrupted square ribs are arranged along the axial flow direction on the bottom wall only. Experimental tests are performed for Reynolds numbers ranging from 8,900 to 29,000. The results are compared with those of previous investigations for circular tube with axial interrupted ribs and twisted tape inserts.