Polydimethylsiloxane (PDMS)-based composites filled with 80, 300, and 700 nm-sized BaTiO 3 (BTO) particles at different concentrations (23, 30, and 40 vol %) were prepared, and their temperature and frequency dielectric properties, as well as electromechanical energy harvesting performance, were studied. BTO/PDMS composites exhibit a low-temperature dynamic glass transition anomaly dependent on BTO size. For 300 and 700 nmsized tetragonal BTO, the glass transition temperature shifts toward higher temperatures as the filler content increases, due to strong interaction between PDMS and BTO. For 80 nm-sized cubic BTO, a weak interaction between fillers and matrix produces an inverse temperature shift. The electromechanical response in terms of energy harvesting of BTO/PDMS-based nanogenerators, tested under a periodic vertical compression force of 20 N at frequencies of 1-15 Hz, showed an optimal BTO concentration at a certain particle size. The best output parameters were found for 23 vol % of 300 nm-sized BTO, namely, 9.99 V and 2.17 μA for voltage and current, respectively. Two different types of the electromechanical response were detected. Electrostriction and dielectric elastomer capacitor phenomenon are the basic mechanisms by which energy harvesting is explained in pure PDMS and composites with 80 nm-sized BTO. Whereas for the composites with 300 and 700 nm-sized BTO, the piezoelectric effect is responsible for the output performance. This work not only offers a solution for energy harvesting applications but also provides data on dielectric relaxation processes and electromechanical conversion capability as a function of filler size and content.