A hydrophobic layer was formed on smooth surfaces of Calotropis gigantea fiber (CGF) and kapok fiber (KF) by adsorption of octadecyltrichlorosilane (OTS) from a toluene solution and then a comparative study was carried out on the basis of various characterizations and oil-absorbing performances for the two natural plant fibers. The resulting OTS-CGF and OTS-KF exhibit outstanding hydrophobic–oleophilic property and an enhancement in the oil-absorbing capacity for engine oil, soybean oil and kerosene. Moreover, the fibers can be utilized for rapid and selective removal of oil spills on the water surface. Compared to KF, CGF seems to be acid-resistant during the hydrolysis process of OTS, with the result that the oil-absorbing capacity exhibits no significant decrease after ten cycles. Eventually, CGF-based material can be further developed for oil–water separation, demonstrating its potential as a promising alternative for treatment of oil-containing wastewaters.