In this paper, flame characteristics of ethylene and its mixture with methane and hydrogen in process of ignition in a Mach 2.5 supersonic model combustor are studied. Development of flame structure is visualized with CH* chemiluminescence images recorded by high-speed camera. The dominant frequencies associated with flame oscillations are identified via fast-response pressure measurements. The present results show that there are two typical flame structures of triangle-shape and ellipse-shape found at relatively low and high fuel-to-air equivalence ratios respectively. The triangular flame at low equivalence ratios is mainly located in the shear layer of cavity flow and has small oscillations featured with low frequencies. At high fuel-to-air equivalence ratios, flame moves upstream and is anchored at the front edge of the cavity with elliptic shape and larger oscillations. The wall pressure distributions indicate that shock structures are formed in the isolator upstream of fuel injections at high equivalence ratios, leading to low speed flow in vicinity of fuel injections and causing flame move forward and change shape. Dominant frequencies from 49 Hz to 317 Hz are found for ethylene or mixture fuels. As fuel-to-air equivalence ratio increases, the dominant frequencies become smaller. For mixture fuel with hydrogen and methane, similar ignition process and flame structures are observed. However, the dominant frequencies related to flame oscillations for mixture fuels are not the same due to different combustion performance and thermal dissipation on turbulent flow. (C) 2019 Published by Elsevier Masson SAS.