Serviceability limit state is one of the required criteria that must be met in any analysis and design procedure. All concrete design codes have focused on the calculation methods for these criteria. One of these criteria is the deflection, which depend on the flexural stiffness (EI) of the studied element. Two methods for determining the effective flexural stiffness were studied: Branson’s method that was derived in 1965, and Bischoff’s method that was derived in 2005. Then these methods were used to verify an experimental work conducted for reinforced concrete beams with light weight aggregates. It has been found that the load deflection behavior of reinforced concrete beams with light weight aggregates were different from that with normal weight. A theoretical equation for each beam with lightweight aggregates was derived and compared with the real experimental curve. Finally, conclusions and recommendations were stated.