This chapter discusses the effect of composition of CNF reinforced nanocomposites on their mechanical properties measured under tensile, compressive, and flexural loading. The structure of the CNFs plays an important role in determining the reinforcement efficiency. In most cases, random CNF dispersed nancomposites have been studied. Thermoplastic resin, thermosetting resin, and elastomer matrix nanocomposites have been studied. Polypropylene is the most common thermoplastic resin that is reinforced with CNFs. Among thermosets, epoxy and vinyl ester resin matrix nanocomposites are studied. The strength and stiffness improve, depending on the volume fractions of CNF dispersed within several different polymer matrices. It is noted that compared to the mechanical properties of single CNFs reported in Chap. 2, the level of enhancement of mechanical properties of nanocomposites is only moderate. The bending of long aspect ratio fibers and stacked-cup structures are potential reasons for this outcome. Nevertheless, combined with other properties, such as electrical and thermal conductivity increase in otherwise insulating resins, the moderately enhanced nanocomposites can develop new applications.