Abstract: Simultaneous OH and CH₂O Planar Laser-Induced Fluorescence (PLIF) imaging plays an important role in studying flame structures and two-dimensional distribution of intermediate species in combustion. The OH distribution is used to represent the reaction zone structure of flames, whereas CH₂O is employed to show the pre-heating zone structure of flames. Flame structures of a CH₄/air partially premixed flame were investigated using simultaneous OH and CH₂O planar laser-induced fluorescence (PLIF) imaging. Experimental methods, including experimental system, optic alignment, synchronization scheme, excitation scan of OH A-X(1, 0), data collection and analysis, are discussed. Experimental results suggest that simultaneous OH and CH₂O PLIF imaging can show different instantaneous structures of reaction zones and preheating zones in the CH₄/air partially premixed flame. New separated preheating zones are locally generated due to the merge of the reaction zone between two adjacent areas.