NOx and CO emissions of fuel rich-lean flame of natural gas water heaters were experimentally investigated. Fuel-rich and fuel-lean flame with different air factors were analyzed separately. Emission of fuel-rich flame is CO whereas emission of fuel-lean is NOx, and fuel rich-lean ratio is the most important factor for NOx and CO emission. If fuel-rich flame α1 is changed with constant fuel-lean flame α2, NOx emission decreases and CO emission increases evidently. If α2 is changed with constant α1, NOx decreases slightly and CO increases initially and then decreases. Depressing fuel-lean flame can strengthen excess O2 diffusion to fuel-rich flame, and then reduces NOx and CO emissions.