A computational study using M06 method1,2 on the cyclopropanation of endo-dicyclopentadiene with Simmons-Smith zinc carbenoids is reported.Each channel proceeds via the methylene-transfer mechanism.The channels with IZnCH2I attacking the double bonds from the exo-face have much lower barrier (about 16.17-18.43 kcal/mol) in gas phase, compared with the channels from the endo-face (21.80-31.13 kcal/mol).