disclosed is a process for preparing 3 , 4 , 5 , 6 - tetrahydrophthalimide comprising the steps of : reacting a crude 3 , 4 , 5 , 6 - tetrahydrophthalic anhydride containing as impurities at least one carboxylic anhydride selected from the group consisting of phthalic anhydride , hexahydrophthalic anhydride and structural isomers of 3 , 4 , 5 , 6 - tetrahydrophtalic anhydride with an alcohol for conversion of substantially all the carboxylic anhydride into the monoester thereof to obtain a reaction mixture containing the monoester , 3 , 4 , 5 , 6 - tetrahydrophthalic anhydride , and if any , a monoester of 3 , 4 , 5 , 6 - tetrahydrophthalic anhydride , and adding ammonia to the reaction mixture to subject the 3 , 4 , 5 , 6 - tetrahydrophthalic anhydride , and if any , the monoester thereof to a reaction for conversion into 3 , 4 , 5 , 6 - tetrahydrophthalimide .