Pure and rare-earth metal [Yttrium (Y), Ruthenium (Ru) and Caesium (Cs)]-doped ZnO thin films were deposited onto In-doped SnO2 substrates by chemical bath deposition. The present study detailed investigated the effect of rare-earth metal-doped ZnO on NH3 gas sensing. X-ray diffraction analysis indicated that the incorporated rare-earth metal ions substitute Zn sites in the ZnO lattice. Dimension of ZnO films decreased with rare-earth metal doping which detected from surface morphology images. The response of 100 ppb NH3 gas was calculated to be 0.80 (200 degrees C), 14.00 (90 degrees C), 17.00 (50 degrees C), and 10.00 (120 degrees C) for the pure, Y-, Ru-, and Cs-doped ZnO films, respectively. In addition, the response of 15 ppm NH3 gas at room temperature was calculated to be 0.20, 27.00, 57.00, and 18.00 for undoped Y-, Ru-, and Cs-doped ZnO films, respectively.