AbstractRecently, nodal line semimetals based on ZrSiS-family have garnered massive research interests contributing numerous experimental and theoretical works. Despite being the most studied nodal-line semimetal, a clear understanding of the transient state relaxation dynamics and the underlying mechanism in ZrSiS is lacking. Using time- and angle-resolved photoemission spectroscopy, we study the ultrafast relaxation dynamics in ZrSiS and reveal a unique relaxation in the bulk nodal-line state which is well-captured by a simple model based on optical and acoustic phonon cooling. Our model predicts linear decay processes for both optical and acoustic phonon relaxations with optical cooling dominant at higher temperatures. Our results reveal different decay mechanisms for the bulk and surface states and pave a way to understand the mechanism of conduction in this material.