A green approach has been developed for the synthesis of nanocrystalline zinc oxide (ZnO) with the aid of room-temperature synthesized ionic liquids (RTIL's) as crystal growth modifiers by low-temperature precipitation technique. The role of RTIL's (propylammonium acetate (PAA), propylammonium formate (PAF), 3-hydroxy propylammonium acetate (3-HPAA), 3-hydroxy propylammonium formate (3HPAF) and their concentration effect on the particle size is studied in this protocol. The formed nanoparticles are characterized by XRD, TEM, FT-IR and UV-DRS. XRD spectra of nanoparticles exhibit typical diffraction peaks of hexagonal phase with wurtzite ZnO structure corresponding to JCPDS 36-1451. TEM results revealed that spherical nanoparticles obtained with an average particle size in the range of 5-20 nm. UV-Vis-DRS spectra of the ZnO nanoparticles shows blue shift compared to the bulk ZnO, attributed to quantum confinement effect.