Microstructural characterization of nylon 6/short glass fiber (SGF) and nylon 6/polyacrolonitrile based carbon fibers (PAN-CFs) of 10 to 40 wt% has been performed by positron lifetime technique (PLT). The positron lifetime parameters viz., o-Ps lifetime ((3)), o-Ps intensity (I-3), and fractional free volume (F-v) of nylon 6/SGF and nylon 6/PAN-CF composites are correlated with the mechanical properties viz., tensile strength and Young's modulus. The F-v shows negative deviation with the reinforcement of 10 to 40 wt% of PAN-CF and show positive deviation in nylon 6/SGF from the linear additivity relation. The negative deviation in nylon 6/PAN-CF composite suggests the induced molecular packing due to the chemical interaction between the polymeric chains of nylon 6 and PAN-CF. The positive deviation in nylon 6/SGF composite indicates the formation of interface between the polymeric chains of nylon 6 and SGF. The increased crystallinity of nylon 6/SGF and nylon 6/PAN-CF composites shows the improved mechanical properties of the composites. The hydrodynamic interaction parameter (h), which shows more negative values in nylon 6/SGF than nylon 6/PAN-CF composites. However, the extent of chemical interaction in nylon 6/SGF is less compare to nylon 6/PAN-CF composites. This is evident from Fourier transform infrared spectrometry studies. POLYM. ENG. SCI., 58:1428-1437, 2018. (c) 2017 Society of Plastics Engineers