Flame treatment is a common technique to modify the surface of polymer materials. Because of local heating this treatment can modify the structure of semicrystalline polymers. In this contribution we investigate how different flame treatments affect the morphology of the surface layer of coextruded PA-PP pipes with PA as outer layer. For this purpose test specimens have been analyzed by fast scanning chip calorimetry (Flash DSC 1 from Mettler-Toledo). The samples are taken from the outer 150 μm part of the PA layer. To reduce reorganization during heating, rates of 1000 K/s and higher have been applied. The measurements show that as a consequence of the flame treatment crystallinity changes. Also we could show that the flame treatment changes the average crystal size in the investigated distance to the surface. The DSC curves can be also used to estimate the average temperature in the surface region of the pipes during the flame treatment. The modification of the morphology in the surface region also affects the mechanical properties of the PA-PP-pipes.