Chemical bath deposition (CBD) is an attractive thin film deposition technique for depositing compound semiconductors at low temperature. In this paper, nickel oxide thin films were prepared by CBD from an aqueous solution of NiSO4, K2S2O8, and ammonia at room temperature. The growth mechanism was studied by quartz crystal microbalance, UV-Vis absorption, and photon correlation spectroscopy. The data indicated the film growth is strongly depending on mixing conditions and accompanied by a homogeneous reaction of particle formation. No film formation could be observed without the addition of persulfate. The growth mechanism is most likely a combination of particle sticking and molecule-by-molecule growth. The obtained thin films have an optical band gap value of 3.5 eV estimated from UV-Vis absorption. The film surface has a sponge-like morphology according to scanning electron microscopy. The as-deposited film contained α-Ni(OH)2 and was converted to NiO by thermal annealing according to thermogravimetric, X-ray diffraction and X-ray photoelectron spectroscopy analysis.