DMTMM was used as condensing agent for synthesis of 1,2,4-oxadiazoles from corresponding carboxylic acids and amidoximes in the mixed solvent of THF and NMP. First, carboxylic acids were activated by DMTMM, and then reacted with amidoximes to convert to O-acylated amidoximes. The unusual solvent system THF-NMP successfully promoted dehydration process of O-acylated amidoximes and 1,2,4-oxadiazoles were acquired in one-pot. This methodology provides a convenient and facile synthesis of 1,2,4-oxadiazoles with high yield.