YBCO samples with different microstructures were prepared after the thermal treatment of a precursor previously obtained by autocombustion. A drastic influence of the particle size on the magnetic behavior of the samples was observed. Thus, particles smaller than 110 nm do not exhibit superconducting properties and for those ranging around 200 nm the diamagnetic signal characteristic of the superconductivity at low temperature disappears in a large applied magnetic field. Particles larger than 300 nm do not exhibit the particle size effect. Accompanying such a phenomenon, an increase of the superconducting critical temperature is observed with the augmentation of the particle size, the lowest value being 18 K which corresponds to 110 nm particles.