A new method to measure accurately the compression ratio of automobile engines in the manufacturing line is proposed. To determine the compression ratio of an engine, the cylinder volume at a piston position is measured with a detecting probe which is featured with a pressure sensor of high sensitivity and a micro piston driven by an electromagnetic coil. The probe is attached to the cylinder head at the spark plug hole. The air in the cylinder at TDC is compressed sinusoidaly at a frequency of 70Hz by the micro piston with a diameter of 5mm. The volume at TDC can be determined by detecting the pressure change caused by the compression by the micro piston.In this report, the principle of the method is described and effects of geometry of the chamber space and other parameters on the measuring accuracy are discussed. Polytropic exponent of the pressure change was found most crucial in this method. Theoretical and experimental analysis show that the polytropic exponent mainly depends not on the volume but on the volume/surface ratio of the chamber.The effects of the material characteristic of the cylinder wall and roughness of the surface on the pressure change were also investigated experimentally.Applying the prototype detecting probe and using a calibration vessel with a known volume, an overall accuracy of 0.5% was obtained.