Using steam as a heat carrier and working media has merits to increase electric efficiency up to 60% and decrease NOx emission to single-digit compared to dry gas turbine cycles. These attribute primarily to the physical properties of steam as having high heat capacity to reduce local flame temperature, and hence reduce emissions by inhibiting the thermal NOx forward reaction rate. In this work, ultrahigh steam content with a steam-to-air mass ratio of up to 40% is premixed with methane–air mixture before entering into a swirl-stabilized high pressure (HP)-burner for combustion. A significant change of flame from V-shape (attached) to M shape (detached) is observed through a transparent combustion chamber whilst changing steam content. The measurement of chemiluminescence OH* is conducted with an intensified CCD-camera bandpass filtered at 320 nm. Following these measurements, large eddy simulation (LES) is used to capture reacting flow features. Reasonably well agreements between experimental data and numerical results are obtained for both attached and detached flames in terms of the OH* distribution. Slight inconsistency of OH* intensity is mainly due to uncollected wall temperature, which leads to either over- or underprediction of chemical reaction rate depending on the experimental flame positions. Distributed flame front is clearly identified with LES for wet methane combustion associated with 35% steam-to-air ratio corresponding to a high Karlovitz number flame. Slightly unstable combustion is observed when the steam-to-air ratio exceeds 40% featuring an onset of flame blow-off. In addition, interaction between precessing vortex core (PVC) and the flame is presented for different level of steam dilution, and conclusions are drawn regarding the flame stabilization. The in-depth understanding of the ultrawet combustion is an important step toward the use of sustainable, steam-diluted biosyngas for electricity production.