The planar laser induced fluorescence (PLIF) technique was applied to two dimensional visualization of OH radicals in a combustion flame. A frequency doubled Nd:YAG laser pumped dye laser was used to form a laser light sheet which excited the OH X2Π-A2Σ transition. A fluorescence image of the OH radical and a visible image of a combustion flame were simultaneously imaged by a pair of CCD cameras with image intensifiers. Measurement of the OH radical in the combustion flame could be carried out by using this PLIF technique without Mie scattering lights from soot particles and other optical disturbances.The PLIF technique was employed to study the OH radical in the combustion chamber of a spark ignition (S. I.) engine using gasoline as fuel. Measurements of the OH radical fluorescence were carried out under various operating conditions of mass burned fraction, swirl ratio and air-fuel ratio. Images of the OH radical fluorescence were very complicated in structure compared with visible images of the combustion flame in the engine. The fluorescence distribution of the OH radical was not only observed in the near-flame front but also in the post flame gases. The fluorescence intensity of the OH radical increased with the progress of the combustion. The OH radical fluorescence intensity reached a peak value and the concentration of HC emissions in the exhaust gases indicated a minimum value at the optimum swirl ratio. Both the OH radical fluorescence intensity and the concentration of NOx emissions in the exhaust gases reached a peak value at the near-stoichiometric air-fuel ratio.