ESR study on monodispersed palladium nanoparticles with the mean diameters of 25 Angstrom corresponding to the "magic atom number" of 561 atoms (five-shell nanoparticle) and smaller than 25 Angstrom (22 and 23 Angstrom) was carried out to investigate their electronic structures. Two broad absorption peaks corresponding to Delta S = 1 and 2 and a sharp one with Delta S = 1 were observed in the ESR spectra of palladium nanoparticles. The broad and strongest spectrum with Delta S = 1 may arise from the spin on the large orbital moment state of the nanoparticles. The relatively strong second harmonic spectrum with Delta S = 2 is characteristic of the nanoparticles, and its low-field shift compared with bulk palladium implies the existence of the effective field from the orbital moment of the electron. On the other hand, the sharp ESR signal with Delta S = 1 may be derived from the nanoparticles that have the electronic structure with one s-state electron outside the closed electron shell core with the "magic atom number".