Neurofibrillary tangle (NFT) is a characteristic hallmark of Alzheimer's disease. GSK3Ղ has been reported to play a major role in the NFT formation of tau. Dysfunction of autophagy might facilitate the aggregate formation of tau. The present study examined the role of GSK3Ղ-mediated phosphorylation of tau species on their autophagic degradation. We transfected wild type tau (T4), caspase-3-cleaved tau at Asp421 (T4C3), or pseudophosphorylated tau at Ser396/Ser404 (T4-2EC) in the presence of active or enzyme-inactive GSK3Ղ. Trehalose and 3-methyladenine (3-MA) were used to enhance or inhibit autophagic activity, respectively. All tau species showed increased accumulation with 3-MA treatment whereas reduced with trehalose, indicating that tau undergoes autophagic degradation. However, T4C3 and T4-2EC showed abundant formation of oligomers than T4. Active GSK3Ղ in the presence of 3-MA resulted in significantly increased formation of insoluble tau aggregates. These results indicate that GSK3Ղ-mediated phosphorylation and compromised autophagic activity significantly contribute to tau aggregation.