The main objective of this study is to investigate the flexural and interlaminar shear properties of natural fiber reinforced polymer (NFRP) composites under the effect of mercerization and different volume fractions of the fibers. This study considers fibers from plant bast, namely jute, hemp, and flax, and epoxy resin and hardener, which are matrices that reinforce the fibers in the composite. Three types of fiber reinforced polymer (FRP) composites, namely jute (JFRP), hemp (HFRP) and flax (FFRP), are fabricated in this study. Two sets of composites are manufactured with 0.5 volume fraction of fiber before and after the mercerization. In the same way, another two sets of different composites are fabricated with a 0.4 volume fraction of fiber before and after mercerization. All the composite specimens are cut and tested against the flexural and interlaminar shear tests as per ASTM standards. Scanning electronic micrographic (SEM) studies are used to identify the impact of treatment on the fiber surface and the failure pattern of the composites in both tests. The results showed that the treated composites performed better than the untreated specimens, and the composites fabricated with a 0.4 volume fraction of fiber had the highest flexural and short beam strength. Overall, among all the composites, the treated hemp (HFRP) composite made with a 0.4 volume fraction outperformed all the other composite specimens tested in this study.