Precast prestressed hollow-core reinforced concrete slabs are used extensively for floor and roofing systems in precast concrete construction. This paper investigates reinforced concrete precast prestressed hollow core slabs strengthened with different systems. An experimental test program prepared to test some models of precast prestressed reinforced concrete hollow core slabs before and after strengthening. Nine full-scale hollow core slabs having the dimensions 4100x1200x160 mm were tested. Specimens were classified in three groups. The first group contains two control specimens tested without any strengthening. Second group contains three specimens strengthened by using GFRP strips. Third group contains four specimens strengthened by using steel plates. All slabs were tested under two central the line loads at middle third of span until failure. Test results showed that all of the strengthening techniques are very effective in increasing the element's structural carrying capacity