Over the last two decades, atmospheric-pressure plasma has been investigated intensively as an advanced oxidation process for water. This current study is to focus on the generation mechanisms of H2O2 in the liquid treated by atmospheric-pressure He plasma. The effect of H2O2(aq) on the removal percentage of isopropanol (IPA) in liquid is evaluated after He plasma jet treatment. The effects of IPA as center dot OH(g) and center dot OH(aq) scavenger on the generation of H2O2(aq) are studied by changing the initial IPA solution temperature. Our analysis indicates that H2O2(aq) is mainly formed by the H2O2(g) dissolution into liquid. The combination of center dot OH(aq) is not important for the formation of H2O2(aq) in liquid. When the exogenous H2O2 is added into IPA solution, the IPA removal percentage can be improved by the He plasma jet treatment due to the fact that exogenous H2O2 is decomposed to center dot OH by e(-), H radicals, and He and He-2 metastables. This study is important for understanding the reaction pathways of H2O2(aq) in plasma treated liquid.