To　reveal　the　mechanism　of　shear　failure　of　en-echelon　joints　under　cyclic　loading,　such　as　during　earthquakes,　we　conducted　a　series　of　cyclic　shear　tests　of　en-echelon　joints　under　constant　normal　stiffness　(CNS)　conditions.　We　analyzed　the　evolution　of　shear　stress,　normal　stress,　stress　path,　dilatancy　characteristics,　and　friction　coefficient　and　revealed　the　failure　mechanisms　of　en-echelon　joints　at　different　angles.　The　results　show　that　the　cyclic　shear　behavior　of　the　en-echelon　joints　is　closely　related　to　the　joint　angle,　with　the　shear　strength　at　a　positive　angle　exceeding　that　at　a　negative　angle　during　shear　cycles.　As　the　number　of　cycles　increases,　the　shear　strength　decreases　rapidly,　and　the　difference　between　the　varying　angles　gradually　decreases.　Dilation　occurs　in　the　early　shear　cycles　(1　and　2),　while　contraction　is　the　main　feature　in　later　cycles　(3−10).　The　friction　coefficient　decreases　with　the　number　of　cycles　and　exhibits　a　more　significant　sensitivity　to　joint　angles　than　shear　cycles.　The　joint　angle　determines　the　asperities　on　the　rupture　surfaces　and　the　block　size,　and　thus　determines　the　subsequent　shear　failure　mode　(block　crushing　and　asperity　degradation).　At　positive　angles,　block　size　is　more　greater　and　asperities　on　the　rupture　surface　are　smaller　than　at　nonpositive　angles.　Therefore,　the　cyclic　shear　behavior　is　controlled　by　block　crushing　at　positive　angles　and　asperity　degradation　at　negative　angles.　©　2024　Institute　of　Rock　and　Soil　Mechanics,　Chinese　Academy　of　Sciences