One of the drawbacks of natural fiber-reinforced polymer composites including sugar palm fiber-reinforced polymer composites is their low mechanical properties. In order to improve their mechanical properties, incorporation of natural fiber with higher strength synthetic fiber was used as reinforcement to produce hybrid fiber-reinforced polymer composites. This research aimed at finding out the influence of glass fiber substitution for sugar palm fiber on flexural properties of hybrid sugar palm/glass fiber-reinforced polyester composites. The specimens were cut from composite plates produced using press mold technique, and subjected to a three-point bending test according to the ASTM D790 standard. Whilst the total fiber volume fraction (Vf-tot), was kept constant at 32%, the hybrid ratio (rh), the ratio between glass fiber volume to total fiber volume, was varied at 0.0, 0.1, 0.2, 0.3, 0.4, and the span-to-depth ratio (S/d) was also varied at 16, 24 and 32. The result shows that lateral load deflection relations were found being non-linear up to close to failure. The magnitude of lateral load increases with the increase of glass fiber content. Substitution of 40 vol% of unidirectional glass fiber for sugar palm fiber in tension side can increase the load bearing capacity of the beam specimen up to 364% with respect to randomly oriented chopped sugar palm fiber-reinforced polyester composite specimens. Failures of sugar palm fiber-reinforced polyester composites were initiated at tensile face propagated toward compressive face leading to catastrophic failure. Those of the hybrid fiber-reinforced polyester composite specimens are characterized by separation of glass fiber/polyester layer from sugar palm fiber/polyester layer producing longitudinal in-plane crack followed by breakage of glass fibers.