The isothermal crystallization behaviour of the polypropylene (PP) phase in PP/multi-walled carbon nanotubes (MWCNTs) composites has been investigated via differential scanning calorimetric analysis, which showed the influence of the varying dispersion level of MWCNTs in the respective PP matrix. PP/MWCNTs composites were prepared via melt-blending technique, wherein two different grades of MWCNTs of varying average "agglomerate" size and varying entanglements (N-MWCNTs and D-MWCNTs) were utilized. Furthermore, the influence of melt-viscosity of the PP phase was investigated on the crystallization kinetics of the PP/MWCNTs composites. Heterogeneous nucleation ability of MWCNTs has resulted in a decrease in half time of crystallization (t(1/2)) from similar to 14 min for pure PP to similar to 6 min for PP/N-MWCNTs and similar to 11 min for PP/D-MWCNTs composites at 1 wt% of MWCNTs at 132 degrees C. Overall rate of crystallization (k) has significantly increased to 4.9 x 10(-2) min(-1) for PP/N-MWCNTs composite as compared with 6.2 x10(-3) min(-1) for PP/D-MWCNTs composite at 0.5 wt% of MWCNTs at 132 degrees C. Moreover, the effect of a novel organic modifier, Li-salt of 6-amino hexanoic acid along with a compatibilizer (PP-g-MA) has also been investigated on the crystallization kinetics of the PP phase in PP/MWCNTs composites. (C) 2017 Society of Plastics Engineers