1,3-Dipolar addition of acetonitrile oxide to 2-phenyl-3a,4-dihydro-1-isoindolinone was carried out in a stereoselective but regiorandam manner. However, the introduction of substituent, e. g., methyl group, at the 4-position of the 1H-isoindol-1-one system turned out to improve regioselectivity of the addition. The regio- and stereoselective isoxazoline synthesis was also accomplished in the reaction with other nitrile oxides. The transformation of the isoxazoline ring was also examined.