An experiment was carried out to investigate the effect of the single and double-deck pre-chamber spark plug on the combustion characteristics of premixed CH4/air in a constant volume vessel using schlieren method. A special design was proposed for the visualization of the pre-chamber. Combustion with different initial temperatures (300K, 400K, 500K) were observed at lean to stoichiometric air fuel ratios. The combustion characteristics of premixed CH4/air was characterized by considering both the 0-10% and the 10-90% burn durations of the pressure rise because of combustion. Results revealed that both of pre-chamber designs have the shorter 10-90% burn duration when the initial temperature is 300K, while the 0-10% burn duration is longer. The double-deck pre-chamber exhibited the longer 10-90% burn duration comparing with the single-deck pre-chamber, and the gap is larger as the mixed gas is more lean. For both single and double-deck pre-chamber, the differences of 0-10% burn duration under different excess air coefficient are much smaller than the conventional spark plug. Another interesting finding is that double-deck design has the most widest lean limit . What’s more,the reverse jet (main chamber to pre-chamber) can be observed through the visible pre-chambers.