Polypropylene (PP) nanocomposites were prepared using cellulose nanocrystal (CNC) grafted with 3‐pentadecylphenol (PDP) as a filler. Functionalized CNC (f‐CNC) was prepared by the reaction of CNC with 4‐oxo‐4‐(3‐pentadecylphenoxy)butanoic acid (PDPBA), where PDPBA was synthesized by reacting succinic anhydride (SA) with PDP. PP nanocomposites containing CNC (PP/CNC) and f‐CNC (PP/f‐CNC), respectively, were prepared to study the effect of fillers on PP. PP/CNC and PP/f‐CNC showed improved mechanical stiffness, and especially, PP/f‐CNC was found to have larger mechanical strength value than neat PP and PP/CNC due to the PDP moiety having a long alkyl group in f‐CNC. Antibacterial property was observed both from PP/CNC and from PP/f‐CNC while that of PP/f‐CNC was better than that of CNC because PDP could impart the additional antibacterial activity. The improved mechanical properties and antibacterial property of PP/f‐CNC could be ascribed to the respective effect of PDP and CNC.