The surface characteristics of diamond were modified in KH550 silane coupling agent of alcohol solution and toluene respectively, and the effects of the two methods on the properties of diamond were analyzed. The results showed that the two surface modification methods could realize the chemical combination between KH550 and diamond surface. Compared with the original power, the modified diamond's surface zeta potential changed and diamond's aggregation reduced. Moreover, the amount of silane coupling of diamond surface modified in KH550 toluene solution was more than that modified in KH550 alcohol solution.