Fiber Reinforced Composite (FRC) is a combination material between reinforced fiber and composite resin that serves to distribute the load in order to have better mechanical properties. One of the mechanical properties of FRC is flexural strength. This study aims is to determine the effect of polyethylene fiber position in the compression zone and microporosity to flexural strength of FRC with packable resin composite. This study was experimental laboratory research. Research material used polyethylene fiber (Construct, Kerr, USA) 2 mm of width, packable resin composite (FIltek Z250 XT 3M ESPE, USA), and silane (Vitique, Germany). The sample in this study were polyethylene fiber reinforced composite (FRC) rod shape with a size 25 x 2 x 2 mm with the position of fiber in compression zone 0,2 mm of thickness and overall composite resin 1,8 mm of thickness. The total number of samples is 4. The application of composite resin is done by using manual condensation technique using condenser and glass slide. The samples were observed using the Scanning Electrone Microscope (SEM) to determine the microporosity (mm2) then immersed in artificial saliva and incubated at 37oC for 24 hours. Then it was tested using the Universal Testing Machine (UTM) to determine the flexural strength (MPa). The result of microporosity and flexural strength obtained were statistically analyzed by One Way Anova test with p=0,008. The results show the average value microporosity of FRC was 0,2588 mm2 and the average value flexural strength of FRC was 99,3975 MPa . These results indicate that there is a significant effect (p <0.05). The conclusion of this research is the lower of microporosity value then the flexural strength will be higher and vice versa.