A new, concise, and efficient method for the synthesis of 3-alkyl-1-isoindolinones was described. 3-Alkyl-3-hydroxy-2,3-dihydro-1-isoindolinones, prepared from the reaction of phthalimide and alkyl lithium, were treated with sodium cyanoborohydride in acidic medium to concomitantly undergo dehydration and reduction leading to various 3-alkyl-1-isoindolinones in good yields.