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The Louisiana Department of Transportation and Development currently uses empirical “book ” values in the overlay thickness design. This can lead to errors since the values do not necessarily represent actual field conditions. The objective of this study was to establish an overlay thickness design method for flexible pavement in Louisiana based on in-situ pavement conditions and non-destructive test methods. Four overlay rehabilitation projects with different soil subgrade types and traffic levels were selected for this study. On each pavement section, nondestructive deflection tests (NDT) including Falling Weight Deflectometer (FWD) and Dynaflect were performed at a 0.1-mile interval, and a detail condition survey data including cracking, rut depth, International Roughness Index (IRI), mid-depth temperature, and pavement thickness was also collected. Four NDT-based overlay design methods were investigated an