In this paper, the effects of excess air ratios (lambda) on nitric oxide (NOx) emissions of a hydrogen-fueled spark ignition engine in the cold start period are studied. Cold start characteristics of hydrogen-fueled engine were investigated experimentally. The study was performed under different lambda. The experimental results showed that, when lambda declined from 1.6 to 0.7, the peak engine speed within the first 6 s increased and in-cylinder pressure in the first cycle raised firstly then decreased slightly while the flame development and propagation periods shortened, and the exhaust temperature at the 6th s raised from 329 K to 355 K. In addition, NOx emissions obviously decreased, whereas hydrocarbon (HC) and carbon monoxide (CO) emissions caused by the evaporated lubricant oil increased by decreasing lambda within the first 6 s. (C) 2018 Published by Elsevier Ltd on behalf of Hydrogen Energy Publications LLC.