The nanoparticles of an azo transition metal chelate compound, 2-(2-thiazolyazo)-5-diethylaminophenoI nickel chelate (Ni-TAEP), with a series of sizes were prepared using the reprecipitation method. The enhanced and size-tunable off-resonant third-order nonlinear optical (NLO) responses of these nanoparticles were confirmed. The NLO enhancement of the nanoparticles compared to that of the monomer was demonstrated to originate from the increased Intra-CT process due to the improved planarity of the ligands in the particles. The size-dependent NLO responses of the nanoparticles were attributed to the gradual predominance of the Inter-CT at the expense of the Intra-CT with increasing the particle size. (C) 2004 Elsevier B.V. All rights reserved.