Composites of natural rubber (NR) reinforced with short carbon fibers (SCF) were prepared with two-roll open mill. Ozone treatment of carbon fibers was used to improve the interfacial interaction between fiber and matrix through chemical bonding and physical interlocking. The effect of treatment time on morphology and chemical component of SCF surface and mechanical properties of composites were thoroughly discussed. Results indicated that ozone treatment increased the surface roughness and the number of oxygen-containing groups. Compared with untreated SCF, tensile strength of NR-loaded SCF oxidized for 2.5 h reached the maximum values 26.9 MPa, and the increasing rate was 44.6%.