In order to transmit a wave efficiently in an electron cyclotron heating (ECH) system, it is important to suppress mode conversion loss caused by coupling in the matching optics unit and misalignment in the transmission line. To understand the cause of mode conversion loss, it is necessary to analyze the mode content in an oversized circular corrugated waveguide. For mode analysis of the propagating wave in the corrugated waveguide, several methods based on the phase-retrieval process and the iterative process are suggested. But, in the Korea Superconducting Tokamak Advanced Research 84-GHz ECH transmission line, a well-known method using burn patterns was used for better coupling of the output beam from the gyrotron onto the axis of the corrugated waveguide by adjusting a large ellipsoidal mirror in an L-shaped chamber, a so-called L-box. During the adjustment of the mirror in the L-box, evidence of the existence of higher modes other than HE11 was found. For the mode content study, the radiation intensity distribution was measured using thermal paper as a function of the distance along the waveguide at a high power level. The mode content of the wave was estimated by comparing the measured burn patterns and calculated patterns at different locations. This paper describes the results of mode content estimation using burn pattern images as a function of the mode mixture ratio.