The effect of cationic starches on removal of nickel and vanadium from crude oils in the presence of microwaves was investigated. A series of cationic starches with different degrees of substitution synthesized by a microwave-dry process were used to remove nickel and vanadium from crude oils. The effects of a number of factors, such as the degree of substitution of cationic starches, microwave time and cationic starches dose on nickel and vanadium removal efficiencies from the crude oils were investigated. The results indicate that the higher the degree of cation substitution on the cationic starch, the greater the effect of electrostatic adsorption of the heavy metal positive ions. Sample CS4 (CS is the abbreviation of the cationic starch) had the best effect on nickel and vanadium removal efficiencies in cationic starch series. The optimum conditions for nickel and vanadium removal from crude oils were as follows: (a) amount of CS4 200 mg/L, (b) microwave power 300 W and (c) microwave time 5 min. Under these conditions, the removal efficiencies of nickel from Iranian and Shengli crude oils were 55% and 60%, respectively, and the removal efficiencies of vanadium were 76% and 79%, respectively. (C) 2010 Elsevier Ltd. All rights reserved.