The structure of turbulent flames was examined in a square-piston engine. Submicron-sized smoke particles were added to the homogeneous propane/air mixture. The Mie-scattering of these particles allowed to visualize two-dimensional flame contours. A copper vapour laser generated pulsed laser sheets at a frequency up to 12 kHz.The flame contours were evaluated with digital image processing methods: fractal analysis, complex Fourier transform procedure and statistical flame contour analysis. The following flame parameters have been deduced applying these methods: Fourier factor, combustion zone thickness, mean radius of curvature, fractal dimension, wrinkling factor, flame-let crossing frequency, integral length scale and flame front thickness.