Lean-burn gas engines have recently attracted attention in the maritime industry, because they can reduce NOx, SOx, and CO2 emissions. However, since methane (CH4) is the main component of natural gas, the slipped methane, which is the unburned methane, likely contributes to global warming. It is thus important to make progress on exhaust after-treatment technologies for lean-burn gas engines. A Palladium (Pd) catalyst for CH4 oxidation is expected to provide a countermeasure for the slipped methane, because it can activate at lower exhaust temperature comparing with platinum. However, a de-activation in higher water (H2O) concentration should be overcome because H2O inhibits CH4 oxidation. This study was performed to investigate the effects of exhaust temperature or gas composition on active Pd catalyst sites to clarify CH4 oxidation performance in the exhaust gas of lean-burn gas engines. The authors developed the method of estimating effective active sites for the Pd catalyst at various exhaust temperatures. The estimation method is based on the assumption that active sites used for CH4 oxidation process can be shared with the active sites used for carbon mono-oxide (CO) oxidation. The molecular of chemisorbed CO on the active sites of the Pd catalyst can provide effective active sites for CH4 oxidation process. This paper introduces experimental results and verifications of the new method, showing that chemisorbed CO volume on a Pd/Al2O3 catalyst is increased with increasing Pd loading in 250–450 °C, simulated as a typical exhaust temperature range of lean-burn gas engines.