To study the effects of fiber surface hydrophobicity on water absorption of composites, ramie fiber was hydrophobically modified with polydopamine/octadecylamine and then used to reinforce polypropylene. The fiber modification was characterized by infrared spectroscopy, scanning electron microscopy, energy spectrometry, contact angle, and water absorption. The mechanical properties and water absorption of the composites were also characterized. The results indicated that polydopamine/octadecylamine was successfully grafted onto the fiber surface. With the increase of octadecylamine concentration, the fiber surface became more and more hydrophobic, and its contact angle achieved a maximum of 118 ± 2.97° when octadecylamine concentration was 8 g/L. However, the modified fiber remained a high level of saturated water absorption of 6.53% ± 0.09%. For composites, when the octadecylamine concentration was 0.5 g/L, its water absorption showed the lowest value of 1.81% ± 0.09%. With more octadecylamine was grafted onto the fiber, the water absorption did not decrease continuously but showed a small growth peak. It is concluded that water absorption of composites is greatly affected by water absorption of fiber. The surface hydrophobic modification on fiber is beneficial to improve its surface hydrophobicity and inhibit the interfacial water absorption between fiber and matrix. However, it fails to the improve internal hydrophobicity of the fiber, nor does it inhibit the fiber in composites from absorbing water. [ABSTRACT FROM AUTHOR]