ATAC (Active Thermo-Atmosphere Combustion) is autoignition combustion in two stroke engines, which occurs by diluting trapped Fuel-Air mixture with residual gas to maintain a high temperature at low load operation. In this study, two-stroke ATAC engine testing was carried out to obtain fundamental knowledge for controlling the autoignition and combustion characteristics in this premixed charge compression-ignition combustion engine. The influences of delivery ratio, equivalence ratio and enginespeed (i.e. compression speed) on autoignition timing, autoignition temperature and combustion duration were investigated. It was found that the ATAC autoignition temperature and combustion duration did not depend on the delivery ratio and equivalence ratio, but were determined by the individual fuel characteristics. Increasing the compression speed reduced the ATAC autoignition temperature a little. ATAC autoignition occured at the crank angle when the mass-averaged temperature of the mixture of residual gas and trapped Fuel-Air mixture reached a certain temperature, characteristic to the fuel used. In other words, the autoignition timing depended on the mass-averaged temperature at port closing timing. But at certain delivery ratios, autoignition did not occur, even though the temperature at this timing was in the autoignition region. This point must be investigated further.