The urea SCR system is a major diesel NOx aftertreatment system. An ammonia slip catalyst (ASC) is located downstream of the SCR system to remove ammonia slipping from it, resulting in forming N₂O having a significantly high global warming potential. Thus, we adopted dual-layered ASCs to identify how N₂O is formed in the oxidation layer and how Zeolite layer thickness effects NH₃ to N₂O conversion characteristics. Engine tests and numerical simulations were conducted. The results show that Zeolite has a supplemental effect of reducing NOx and that an reduction in N₂O formation is possible by increasing zeolite layer thickness while NH₃ conversion is decreased. Thus, it is confirmed that zeolite layer thickness should be optimized to reduce these two emissions.