The objective of this study is to evaluate the effects of existing pavement condition and overlay material property on asphalt concrete (AC) overlay design and performance. The newest AASHTO design software DARWin-ME was used to conduct the analysis under various traffic and structure combinations. The factors considered in the analysis include the modulus of exiting layers, the rut depth of existing layer, the interface condition between AC overlay and existing pavement, and the properties of AC overlay (performance grade and Poisson's ratio). In addition to overlay thickness design, pavement performance analysis was conducted to see the effect of existing pavement condition and overlay material property on individual distresses. Several findings were concluded from this study. First, the sensitivity of overlay design thickness to the condition of the existing AC layer depends on the existing AC layer thickness and design traffic. Second, the existing condition of base and subgrade has no significant influence to the overlay performance and design. Third, the performance analysis results indicate that the modulus of existing AC layer and interface bonding condition have more significant effects on fatigue cracking than on rutting potential.