A series of 40 lightweight reinforced concrete (LWRC) beams of 1400 mm length and a rectangular cross section of 150 £ 200 mm were cast, strengthened and then tested under four-point bending test to study the effectiveness of using externally applied carbon fibre-reinforced polymer (CFRP) composites as a method of increasing the flexural strength of under-reinforced LWRC beams. Parameters investigated include reinforcement ratio, r; r ¼ 0.55r b and r ¼ 0.27r b , CFRP sheet length; 600, 800 and 1000 mm, CFRP sheet width; beam width and half-beam width. Three types of strengthening schemes were used: jacketing covers the beam from bottom and two sides with total width of 500 mm, sheets at the tension side with width equal to beam width and sheets with width equal to half-beam width. Test results showed a limited increase in ultimate load-carrying capacity accompanied with some reduction in mid-span deflection for the strengthened beams. Among the strengthening schemes investigated, jacketing was the most effective for strength enhancement (about 41%) with respect to control beam; however, it reduced ductility significantly. An analytical model was proposed for predicting the ultimate load-carrying capacity of LWRC beams strengthened with CFRP composites.Keywords: carbon fibre-reinforced polymers; lightweight concrete beams; flexure strength; strengthening Abbreviations: r, steel reinforcing ratio; CFRPs, carbon fibre-reinforced polymers; LWRC, lightweight reinforced concrete; F, diameter of reinforcing steel; r b , balanced steel ratio; 1 c , concrete strain; 1 cu , maximum usable compressive strain in concrete; h, overall thickness of a member in mm; c, distance from extreme compression fibre to the neutral axis in mm; 1 fu , design rupture strain of FRP; 1 0 s , strain level in compression steel; d 0 , depth of compression steel in mm; E s , modulus of elasticity of steel reinforcement in MPa; A s , area of tension steel in mm 2 ; A 0 s , area of compression steel in mm 2 ; A f , area of FRP reinforcement in mm 2 ; f 0 c , compressive strength of concrete in MPa; b, width of concrete cross section in mm; b 1 , 0.8 for concrete compressive strength of 35 MPa; c, additional reduction factor of the flexural strength contribution of FRP; X, distance between supports and loading point in m