Due to the urgency of reducing greenhouse gases in the combustion process, low-carbon renewable gaseous fuels are needed to replace the pipeline natural gas. Hydrogen is a competitive sustainable energy source, and also a promising energy carrier because of its zero-carbon property and low molecular weight. This study analyzed the flame characteristics of a cooktop burner operating on different natural gas/hydrogen mixtures. As hydrogen percentage increasing from 0% to 75% in the fuel mixture, more reddish color was observed in the flame, which is against the scientific fact that hydrogen flame color is usually pale blue color or even invisible under normal light condition. Therefore, combustion experiments and electron microscopy analysis are conducted to track the source of the abnormal color in the flame. The results suggest that the abnormal reddish flame color is from hydrogen embrittlement on the storage and transport facilities. (C) 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.