Technology for the enhancement of compression ignition for a natural-gas homogeneous charge compression ignition (HCCI) engine was developed using nonthermal plasma. Specifically, nonthermal plasma was utilized to enhance the ignition of the methane/air premixture by irradiating it in an intake tube. The effect of the irradiation on compression ignition was investigated using a rapid compression and expansion machine; the ignition delay was found to shorten by the influence of irradiation. The dependence of the ignition delay time on the temperature at the end of compression was determined. Chemical analysis of the plasma-processed gas was performed using a gas detection tube as a simple method and ion-attachment ionization mass spectrometry (IAMS) as a novel method. A chemical kinetic simulation was also conducted to examine the temperature dependence of the ignition delay. Aldehydes were found to be the most plausible candidate chemical species that contribute to shortening of the ignition delay.