In internal combustion (IC) engines, in-cylinder flow structure has a great influence on the mixing phenomenon, combustion process and finally on the performance and emission characteristics of the engine. Therefore, understanding in-cylinder flow structure is very vital for optimum engine design. The main objective of this study is to investigate experimentally the ih-cylinder tumble air flow structures in a single cylinder IC engine at various crank angle positions during suction and compression strokes using particle image velocimetry (PIV) at 400 rpm engine speed for a flat piston under motored condition. The two-dimensional in cylinder tumble flow measurements are carried out in the combustion space on a vertical plane passing through the axis of the cylinder. To evaluate the tumble flows, ensemble average velocity vector fields are used and to characterize it, tumble ratios are calculated. This study will be very helpful in understanding the fundamental nature of the in-cylinder tumble flow pattern in an IC engine under real conditions.