Experimental investigations were conducted to study the effect of thermal processing on microstructures of GF/PA6 composites. Different degree of crystallinity and phases of the thermoplastic matrix, and transcrystallinity of the composite interfaces were achieved at different cooling rates during the thermal processing. XRD and DSC results indicated that when the cooling rate was varied from fast to slow, the crystallinity and the ratio of the alpha/gamma phases of nylon6 were increased in the PA6 matrix. The microscopy observations showed that columnar spherulites grew along the glass fibre in the slow cooled thin film samples associated with larger diameter of spherulite structures in the matrix. The columnar spherulite structures around the fibres may be transcrystalline layers and they disappeared with increasing cooling rate during thermal processing. It was also found that samples with large amount of voids and poor interfacial bond was found in the low holding pressure samples during the thermal processing.