Zinc oxide (ZnO) nanoparticles with different particle sizes were synthesized by thermal annealing TA of zinc hydroxide at various temperatures. X-ray diffraction measurements show that all nanoparticles possess a typical wurtzite structure and no other impurity phases are observed. The average particle size (d) increases from 22 to 98 nm as T-A increases from 100 to 850 degrees C. The dielectric properties of nanocrystalline ZnO with various particle sizes were studied in the frequency range of 20 Hz to 1 MHz between 80 K and 320 K. Dielectric measurement indicates that the particle size of samples has great influence on the dielectric behavior of ZnO nanoparticles. The Mu curves show strong grain peaks in all ZnO nanoparticles, whereas only 22 nm and 98 nm particles show weak grain boundary peaks. In addition, the ac conductivity (sigma(ac)) follows Jonscher's power law for all ZnO nano-particles, but two types of conduction mechanisms were observed. It is concluded that the sigma(ac) behavior of 22 nm particles is well accounted by the correlated-barrier-hopping model (CBH), whereas the d > 22 nm particles behavior is probably due to Quantum Tunneling Mechanism (QMT). The different sigma(ac) behavior can be attributed to the various defects in grain boundary and formation of inhomogeneous dielectric structures.