In previous study, a method of combustion detection for homogeneous charge compression ignition (HCCI) using a crank angle sensor and a knock sensor has been estimated . In addition, an ion-current sensor has been used as a countermeasure against abnormal combustion with downsizing and higher compression ratio engines. An ion-current sensor has been newly adopted in engine systems.In this study, detection performance of combustion conditions in HCCI and spark ignition (SI) using with the ion-current sensor was estimated.The purpose of this study was to confirm detectable combustion conditions using with the ion-current sensor, and to confirm a requirement of applied voltage for the ion-current sensor.A detection signal of the ion-current sensor was changed by combustion style (HCCI,SI). Experimental results showed a heat release rate increased with ion signals increasing approximately at the same time in HCCI and SI. It was supposed that ion signals appearing during main combustion period were thermal ion. Although maximum heat release rate of HCCI was higher than one of SI, maximum ion signal of HCCI was lower than one of SI.An experimental result of comparison of detecting ion intensity (II) in HCCI and various combustions (stoichiometric spark ignition, lean spark ignition, internal exhaust gas recirculation (EGR) spark ignition) showed that as maximum in-cylinder temperature was decreased, ion intensity II was decreased. This result showed thermal ion was decreased by lower temperature combustion. This correlation applied a detection of nitrogen oxides (NOx). Therefore, an ion-current sensor becomes an application of NOx sensor.In addition, a timing of maximum in-cylinder temperature was approximately simultaneous with a timing of maximum in-cylinder pressure. Therefore, a detection method of timing of maximum ion signal was able to estimate the timing of maximum in-cylinder pressure. The requirement applied voltage of detecting the timing of maximum in-cylinder pressure was more than 800V, because an ion signal was small in HCCI condition.