Composites containing pure, uniform, and well-dispersed ferroelectric barium titanate (BaTiO3) nanocrystallites with various particle sizes as active phase and polyvinylidene fluoride (PVDF) polymer as matrix are fabricated using a two-step mixing and hot-pressing technique. The dependences of the dielectric and ferroelectric properties of the as-prepared composites on the nanoparticle sizes are investigated systematically. The polarization-electric field hysteresis loops are displayed, indicating the ferroelectricity for the BaTiO3/PVDF nanocomposites. The remanent polarization is found to increase as the size of BaTiO3 particles increases from 25 to 500 nm. The dielectric constant of the composites exhibits a complex variation as a function of the BaTiO3 nanoparticle sizes: it reaches a maximum value in the composites with BaTiO3 nanoparticle sizes of 80-100 nm and decreases again when the particle size is further reduced under 50 nm. (C) 2010 American Institute of Physics. [doi: 10.1063/1.3443582]Natural Science and Engineering Research Council of Canada (NSERC) [203773]; National Natural Science Foundation of China [20725310, 20673085]; National Basic Research Program of China [2007CB815303