The　aim　of　this　study　is　to　investigate　the　influence　of　interface　conditions　on　a　backcalculated　layer　property　for　a　flexible　pavement　with　different　base　types.　The　simple　friction　model　was　utilized　in　the　finite-element　method　(FEM)　to　simulate　the　interface　condition,　and　four　interface　conditions　were　taken　into　account　in　analyses.　The　computed　deflection　time　history　indicated　that　the　inadequate　interface　condition　between　the　surface　and　base　lead　to　the　higher　deflection　peak　for　the　pavement　with　a　cement-treated　base　(CTB)　and　granular　base　(GB),　while　the　interface　condition　between　the　base　and　subgrade　has　a　significant　influence　on　the　deflection　only　for　the　pavement　with　a　GB.　The　deflection　time-history　was　backcalculated　through　a　dynamic　approach　using　the　spectral　element　method　(SEM)　as　a　forward　analysis　engine　in　which　the　layers　were　treated　as　fully　bonded.　The　results　show　that　the　interface　condition　shows　an　apparent　influence　on　the　backcalculated　layer　property.　Due　to　the　different　contributions　of　each　layer　to　the　overall　loading　carrying　capacity,　the　interface　condition　influence　was　different　for　different　pavement　scenarios,　which　should　be　appropriately　considered　for　backcalculation　and　pavement　evaluation.　(C)　2021　American　Society　of　Civil　Engineers.