For controlling ignition and combustion of an engine fueled by biomass gas, the effect of ozone addition to two different combustion modes was investigated in experiment. Targeted combustion modes are HCCI and Dual-Fuel Engine (DFE). In HCCI, premixture of biomass gas and heated air is compressed and ignited simultaneously in multi points. In DFE, diesel oil is injected and the premixture is burned by flame propagation starting at the ignition of diesel oil. Effect of ozone addition to each combustion mode is compared from the viewpoint of the difference of ignition mechanism. As a result, in HCCI, ignition timing was advanced by ozone addition, and the possibility of controlling ignition by ozone addition was exhibited. On the other hand, in DFE, ignition timing didn't change even with ozone addition. However, combustion type of premixed gas was changed with some amount of ozone added.