The impurity N,N′-dibenzyl-oxalmide in 2,4,6,8,10,12-hexabenzyl-2,4,6,8,10,12-hexaazateracycle［5.5.0.0^5,9.0^3,11］dodecanes (HBIW) prepared by glyoxal and benzylamines was separated by column chromatography and identified by FTIR,¹HNMR, MS and elemental analysis. Futhermore, N,N′-dibenzyl-oxalamide was synthesized with benzylamines and oxalyl chloride,and identified by spectrometric analysis which verified that the impurity was the N,N′-dibenzyl-oxalamide.