Wireless sensor networks (WSNs) are one of the key factors in realizing an Internet of Things (IoT) society. Piezoelectric materials that can convert mechanical energy into electric energy directly are highly promising as energy-harvesting materials for supplying power to wireless sensors. However, it is well known that lead zirconate titanate (PZT), which has the highest piezoelectric performance, is harmful to the environment. Moreover, the high density and brittleness of the piezoceramics hinder the fabrication of a long-life energy-harvesting device. In this study, we fabricated 0-3 structure piezoelectric composite materials consisting of lead-free piezoceramic nanoparticles, epoxy resin, and carbon-fiber-reinforced polymer (CFRP) and evaluated their mechanical properties. The maximum strain energy of the piezoelectric resin/CFRP composites was 20–40% larger than that of piezoelectric/epoxy composites. The maximum stress and flexural modulus of the lead-free piezoelectric potassium sodium niobate (KNN) nanoparticles/CFRP composite were approximately 5–10% larger than those of the barium titanate (BTO) nanoparticles/CFRP composite. Consequently, it is likely that better energy harvesting performance and mechanical properties can be obtained by using KNN nanoparticles than by using BTO nanoparticles.