Abstract Here we show a simple method for the preparation of modified nucleoside triphosphates using nucleoside-diphosphate kinase. 8-Hydroxy-2′-dGDP and 2-hydroxy-2′-dADP were incubated with ATP and nucleoside-diphosphate kinase. By the transfer of the γ-phosphate of ATP, the cognate triphosphates were formed. This method is applicable to the preparation of modified nucleoside triphosphates that are labeled at the γ-phosphate and/or are difficult to purify.