In previous reports, robustness control logic for HCCI engine using crank angle sensor and knock sensor was discussed. Recently, ion current sensors have been started to use as a countermeasure against abnormal combustion for downsizing engines and high compression ratio engines. In a view point of the trend mentioned above, application of the ion current sensors to robustness control for HCCI was examined. In case of applied voltage 225V, correlation with indicated mean effective pressure, maximum pressure rise rate and ion signal were low. The cause was estimated that cylinder temperature (thermal ion) of HCCI was lower than one of spark ignition combustion.