The improvement of fuel consumption is the most important issue for engine manufactures from the viewpoint of energy and environment conservation. A piston-cylinder system plays an important role for the reduction of an engine friction. For the improvement of the frictional behavior of the piston-cylinder system, it is beneficial to observe and analyze the frictional waveforms during an engine operation.To meet the above-mentioned demand, frictional waveforms were measured with using the renewed floating liner device. In the newly developed floating liner device, an actual cylinder block itself was used as a test specimen.The measured single cylinder was an aluminum monolithic type made of hypereutectic Al-17%Si alloy using a high pressure die casting process. The combined piston was a light weight forged piston and a DLC coated piston ring was used. For the measurement, 110cc air cooled single cylinder engine was used.The observed waveforms were considered to be reasonable and proper from the theoretical point of view. A friction mean effective pressure (FMEP) value was also calculated on each measurement conditions. The FMEP was increased with increasing a load of an engine operation. It was considered that the increase of the FMEP was attributable to the increase of a side force of the piston. Furthermore it was proved that the FMEP was decreased through its running-in process. The FMEP after running-in was 30% lower than that before running-in. This phenomenon was due to the change in the roughness of contact surfaces. And also the tribofilm formed on the contact surface would contribute to the reduction of the FMEP.