This paper presents results from a study carried out on lightweight concrete to ascertain the size effect on reinforced concrete deep beams when lightweight aggregates are used. Sixteen deep beam specimens, some made of sand-lightweight concrete (SLWC) and others made of all-lightweight concrete (ALWC), were tested and compared with normal weight concrete (NWC) deep beams. The shear span-to-overall depth ratios investigated were 0.5 and 1.0, and the section overall depth varied between 400 mm and 1000 mm (15.7 and 39.4 in.). Results showed that the size effect in lightweight concrete (LWC) deep beams was more significant than in NWC deep beams. The effect of section overall depth on the ultimate shear stress was slightly greater in ALWC deep beams than in SLWC deep beams. The conservatism of strut-and-tie models (STMs) proposed by ACI 318-08, CSA, EC2, and Tan and Cheng (2006) is established against the variation of section overall depth in LWC and NWC deep beams without shear reinforcement. The ratio between predicted and measured shear strengths for LWC deep beams is higher than the ratio for NWC deep beams in all four STMs. The findings also show that although Tan and Cheng’s model more closely agrees with the test results for NWC deep beams, it overestimates the shear strength of LWC deep beams.