The physical and optical properties viz., water absorption pattern, density, color and opacity of oil palm fiber-LLDPE composites were studied. The effect of fiber size, fiber loading and fiber treatment on the above parameters was also studied. Alkali treatment on fibers was done to reduce the hydrophilic nature of composites. It was found that the water absorption in most of the combinations followed typical fickian behavior. The rate of water absorption and swelling increased with fiber loading. However alkali treatment on fibers resulted in reduction of water absorption at higher fiber loading only and composites with higher fiber size exhibited higher water absorption. True density of oil palm fiber-LLDPE composites were in the range of 967-1177 kg m-₃, whereas the bulk density ranged from 942-1122 kg m-₃. The dielectric constant of the composite was in the range of 3.22 to 6.73. The L*, a* and b* values of oil palm fiber-LLDPE composites were respectively in the range of 42.25-59.69, 5.85-6.30, 15.65- 22.06. Opacity of LLDPE specimen of 3 mm thickness was 63.58%. Predicted values of true density of composites were in good agreement with measured values.