FRC is a combination material of polymer matrix and reinforced fiber. Reinforced fiber serves to distribute the load from the matrix to the fibers and it can improve the mechanical properties of FRC (flexural strength). The position of fiber on tension zone and the microporosity has an important influence of FRC flexural strength. The aim of this study was to determine effect of polyethylene fiber position (tension zone) and the microporosity on FRC flexural strength (Packable Resin Composite). This study was an experimental research laboratory. Materials research used Polyethylene fiber 2mm width, Filtek Z250XT Packable, silane, and artificial saliva. The sample in this study were the polyethylene fiber reinforced composite (FRC) with 25x2x2mm and the fiber is in the tension zone of 0.2 mm thickness and the entire thickness of packable resin composite is 1.8 mm. Resin was applied by manual condensation technique using condenser and glass slide. The samples were observed using SEM to determine the microporosity then the samples were immersed in artificial saliva and incubated at 37°C for 24 hours. The samples were tested using UTM to determine the flexural strength with the 1 Newton initial load and the fulcrum of the sample is 2 mm. The results of statistical test of microporosity and flexural strength by using One Way Anova method (p=0,000). The result show the average value of FRC microporosity is 1,68 mm2 then the FRC flexural strength is 99,32 Mpa. These results show a significant effect (p <0.05). The conclusion of this research is the lower value of microporosity then the flexural strength will be higher and vice versa.