Land ll leachate contaminated groundwater is widespread all over the world. In order to study the organic contaminant removal e ciency of land ll leachate contaminated groundwater under oxygen micro-nanobubble (MNB) aeration, a series of lab-scale experiments of oxygen MNB aeration as well as common bubble (CB) aeration were conducted. On the one hand, the difference of mass transfer, microbial activity enhancement and contaminant removal e ciency between MNB and CB aeration were estimated. On the other hand, the composition variations of dissolved organic matter (DOM) in groundwater treated by MNB or CB aeration were characterized by ultraviolet-visible (UV-VIS) absorption spectrum and uorescence excitation-emission matrix (EEM). The test results showed that the oxygen utilization e ciency and volumetric oxygen transfer coe cient of MNB aeration were 10 and 50 times that of oxygen CB aeration, respectively. On the 30th day after MNB aeration, the dehydrogenase activity (DHA) of groundwater increased by 101.25%. Compared with CB aeration, the chemical oxygen demand (COD), 5-day biochemical oxygen demand (BOD 5 ) and ammonia nitrogen removal e ciency under MNB aeration increased by 29.72%, 13.43% and 138.59%, respectively. With the biodegradation effect of MNB aeration, a large number of protein-like and soluble microbial by-product substances were degraded, and humic and fulvic acid-like substances were degraded to a certain level. Oxygen MNB aeration played a chemical oxidation effect while enhancing the biodegradation of groundwater, and it was an energy-e cient land ll leachate contaminated groundwater treatment method.