Engine cylinder pressure is one of the most important parameters for design and diagnostic purposes. In-cylinder pressure transducers are one of the most popular measuring sensors needed for combustion analysis in internal combustion engines where they provide one of the most direct effective measures of combustion quality not only on averaged cycles basis but also on cycle by cycle resolved basis. Due to the high cost of using pressure transducers with amplifiers and requirements of continuous cooling, the need for another method of predicting cylinder pressure has been under investigation for many years. Ion current concept is one of the new advanced techniques which was adopted in the last few years to be one of the most useful alternative methods and can substitute the conventional pressure transducers. In the present paper the in-cylinder pressure was estimated using the data of the ionization current collected from the active spark plug using negative polarity. The results showed that certain features of the ion current may be used in conjunction with the appropriate algorithm to approximate cylinder pressure and its related combustion parameters. Also, it was revealed that the negative potential provides an accurate simple method for pressure estimation. By using the new advanced techniques, the possibility of successfully utilizing ion current features for estimating the cylinder gas pressure increases for real time engine control and for on-board diagnostics.