The experimental investigation of the effect of fuel additives on HCCI combustion process was carried out on a 4-cylinder diesel engine of which the fourth cylinder was modified to run in HCCI mode. The experimental results indicated that under the steady operating conditions, the ignition timing of the base fuel PRF90 (90% iso-octane+10% n-heptane), which is equivalent to RON 90, delays and the peak value of the heat release rate of the base fuel is low compared with the base fuel with additives which advances ignition timing and increases the peak of the heat release rate obviously. At the conditions with the same fuel injection quantity per cycle and the same engine speed, with the increase of the additive mass fraction in the fuel, the ignition and heat release timings bring forward and the peak of heat release rate increases. However, excessive additives will make the engine be prone to knock and run rudely.