An experiment was carried out in a direct injection spark ignition (DISI) engine to research the effect of aromatics, especially the heavy aromatics (aromatics with c >= 9) content on the characteristics of in-cylinder combustion and engine-out ultrafine particle. The results show that total aromatics and heavy aromatics content all have a limited effect on in-cylinder combustion. But the ultrafine particle, especially the nucleation mode particles, increases with the increment of aromatics content under warm engine operation condition. More importantly, higher total aromatics content does not always lead to more ultrafine particles and it is dependent on the content of heavy aromatics, which means that the heavy aromatics content has a more significant effect on ultrafine particles. On the contrary, total aromatics content as well as heavy aromatics content has a limited effect on ultrafine particles under cold engine operation condition which emits much more particle number (PN) than it emitted under warm condition. In addition, PN increase significantly with the increment of aromatics content at warm idle operation condition, but no matter total aromatics or heavy aromatics has a limited influence on PN at cold idle operation condition. And it also shows that the effects of aromatics on particle size distribution and PN are affected by excess air coefficient. The decrement of excess air coefficient would enhance the effect of aromatics on PN. (C) 2017 Elsevier Ltd. All rights reserved.