Flame treatment is a common technique to modify the surface of extruded polymer pipes. 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 ìm 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. We could also show that the flame treatment changes the average crystal size in the investigated layer of the pipe. The DSC curves can also be used to estimate the average temperature at the surface of the pipes during the flame treatment. The modification of the morphology at the surface also affects the mechanical properties of the PA-PP-pipes.