In recent decades, engineers have focused on finding solutions to reduce the weight of concrete structures. Undoubtedly, the coarse aggregate weight in concrete is important. This study examined the flexural behavior of zero coarse aggregate concrete with Glass Fiber (GF) added to the steel reinforcement. Also, normal-weight fine aggregate was substituted with autoclaved aerated concrete (Thermostone) by 50% and 75% by weight. The process involved comparison of the test results of two groups. The first group comprised normal reinforcement Lightweight Aggregate Concrete (LWAC), while the second group comprised fiber-reinforced LWAC and a specimen of Lightweight (LW) mortar. Fiber addition boosts energy absorption and slows down the rapid development of crack formation. GFs by 1.5% of concrete weight were added. The results revealed a decrease in the failure load of beams reinforced with GF compared to those reinforced with steel bars. The decrease amounted to 54%, 50%, and 59% for aggregate replacement percentages of 0%, 50%, and 75%, respectively. Replacing steel reinforcement with GF reduced the ultimate load by almost half. All beams with steel reinforcement experienced flexural failure, while the beams with GF reinforcement underwent shear failure.