The relation between shear-mixing time in the melt, polymer crystal size, and electrical conductivity is studied for high density polyethylene carbon nanotubes nanocomposites with the help of in-situ rheo-dielectric and rheo-small angle synchrotron X-ray scattering (SAXS) techniques. Results show that the memory of crystal structure obtained after melt-mixing is not easily removed by re-melting and annealing. The conductivity of composites shear-mixed for 20 min cannot reach that of 10 min samples, even after additional quiescent annealing for 6000 s at 190 degrees C. The rheo-SAXS data further indicate that the temperature needed to melt all the crystals is higher for the longer time shear-mixed composites. These results all suggest that with long processing time, larger crystals are nucleated and grow on the nanotubes, which reduce electrical conductivity, presumably because they prevent electrical contact between the nanotubes. (C) 2012 Elsevier Ltd. All rights reserved.