This article reports the rheological behavior of nanocomposites of isotactic polypropylene with both unmodified multiwall carbon nanotubes (CNTs) and phenol and 1-octadecanol (C18) functionalized CNT (f-CNT) at 0.1, 0.25, 1.0, and 5.0 wt% of the nanofillers. The incorporation of CNT at low loadings of 0.1 and 0.25 wt% decreases the storage and loss modulus and complex viscosity of the system, especially for the case of using f-CNT. Out of the two types of functionalizations, C18 functionalization registers the lowest modulus and viscosity and displays processing aid behavior at 0.1 wt% loading, which is believed to be due to the disruption of the polymer entanglements. As the nanofiller loading increases to 1 wt%, the disruption of polymer entanglements effect is balanced by the hydrodynamic effect and subsequently neat polypropylene (PP), and the filled PP systems show similar modulus and complex viscosity. As the nanofiller loading increases further to 5 wt%, the hydrodynamic effect becomes the dominating factor, and the modulus and the complex viscosity of the nanofilled system become greater than that of neat PP. Results suggest that the 0.1 wt% loading of C18 f-CNT could be a useful processing aid additive for improving polypropylene processability. POLYM. ENG. SCI., 52:18681873, 2012. (c) 2012 Society of Plastics Engineers