In-cylinder pressure of an IC engine is considered to be a major source of information about combustion process. It is a generally accepted method to obtain an in-cylinder pressure signal using a pressure sensor (transducer). In this paper, a different approach is presented. Information about the in-cylinder pressure can be obtained by measuring breakdown voltage across the spark-plug gap. A simple means for measurement of such voltage is developed. The breakdown voltage threshold depends on the density of the gas in the combustion chamber. A strong correlation between the measured breakdown voltages and in-cylinder pressure has been established.