Macroautophagy (herein referred to as autophagy) is an evolutionarily conserved self-digestive process cells adapt to starvation and other stress responses. Upon starvation, autophagy is induced, providing cells with needed nutrient supplies. We report here that Unc-51-like kinase 1 (Ulk1), a key initiator for mammalian autophagy, undergoes dramatic dephosphorylation upon starvation, particularly at serine 638 and serine 758. Phosphorylations of Ulk1 are mediated by mammalian target-of-rapamycin (mTOR) kinase and adenosine monophosphate activated protein kinase (AMPK). AMPK interacts with Ulk1 in a nutrient-dependent manner. Proper phosphorylations on Ulk1 are crucial for Ulk1/AMPK association, as a single serine-to-alanine mutation (S758A) at Ulk1 impairs this interaction. Compared to the wild-type ULK1, this Ulk1-S758A mutant initiates starvation-induced autophagy faster at an early time point, but does not alter the maximum capacity of autophagy when starvation prolongs. This study therefore revealed previously unnoticed acute autophagy response to environmental changes.