Polyacrylamide (PAM) hydrogels were reinforced by rod-shaped cellulose nanocrystals (CNCs), which had strong interaction with PAM through the grafting point. Display omitted . ► In situ free-radical polymerization occur on surface of rod-shape biomass cellulose nanocrystals. ► Grafted cellulose nanocrystals act as a multifunctional cross-linker and reinforce polyacrylamide hydrogels. ► The nanocomposite hydrogels show improved compression strength. Rod-shaped cellulose nanocrystals (CNCs) were manufactured and used to reinforce polyacrylamide (PAM) hydrogels through in situ free-radical polymerization. The gelation process of the nanocomposite hydrogels was monitored on a rheometer using oscillatory shear. The chemical structure, morphology, swelling property, and compression strength of the formed gels were investigated. A possible mechanism for forming hydrogels was proposed. The results showed that CNCs accelerated the formation of hydrogels and increased the effective crosslink density of hydrogels. Thus CNCs were not only a reinforcing agent for hydrogel, but also acted as a multifunctional cross-linker for gelation. The shear storage modulus, compression strength and elastic modulus of the nanocomposite hydrogels were significantly improved because of good dispersion of CNCs in PAM as well as enhanced interfacial interaction between these two components. Among the CNC contents used, a loading of 6.7 w/w% led to the maximum mechanical properties for nanocomposite hydrogels.