A laboratory-scale gas turbine model combustor fueled with methane is studied experimentally with the help of three-dimensional computed tomography of chemiluminescence (3D-CTC) and tunable diode laser absorption tomography (TDLAT). In the combustion structure transition, the three-dimensional relative emissions of CH* were measured and taken as qualitative indicators of the heat release rate. This 3D measurement method utilizes 8 multi-directional CH* images as inputs combined with tomographic algorithms to compute the 3D distribution of CH* intensities. The TDLAT system composed of 42 beams (21×21, 21 parallel beams and 21vertical beams). Four water vapor absorption lines, 7185.6 cm-1, 7444.3 cm-1, 7466.3 cm-1, and 6807.8 cm-1, were utilized in each beam using time-division-multiplexed (TDM) method at total measuring frequency of 2.5 kHz. A reconstruction routine based on simulated-annealing algorithm was used to deduce distributions of temperature T and water partial pressure PX. Various lean operating conditions were conducted to examine the impact of flow velocity on heat release rate oscillation and spatial structure transition with an overall equivalence ratio of about 0.65 and dump plane velocity of 2.9-18.3 m/s. During the increase of Reynolds number, the heat release zone and high temperature zone changing obviously along the nozzle radial and axis direction, and the largest heat release plane moves forward significantly.