Numerical study of energetic and emission characteristics of HCCI engine operating on iso-octane based fuel blends comprising hydrogen or syngas as additives is conducted on the basis of 2D CFD simulation for two values of fuel-to-air equivalence ratio phi = 0.4 and 0.2. The features of combustion are analyzed for the case when the maximal mass-average temperature (T-ev)(max) during the combustion of different fuels is realized at an identical crank angle, which can be implemented at adequate intake temperature for every fuel blend. It is shown that the addition of H-2 or syngas retards the ignition and decreases the combustion duration. This effect is more pronounced for leaner mixture with phi = 0.2. The admixture of H-2 to iso-octane leads to the increase of the specific indicated work both for phi = 0.2 and phi = 0.4. The opposite tendency is detected when syngas H-2/CO = 2/1 is added to iso-octane. The total emission index EINOx + EICO is determined mostly by the emission of NOx at phi = 0.4 and by the emission of CO at phi = 0.2. At phi = 0.4, the replacement of iso-octane by iso-octane/hydrogen (syngas) blend increases the NOx emission index (by 86% for iso-C8H18/H-2 = 20/80 blend), which occurs due to the growth of maximal temperature in the cylinder. At phi = 0.2, the addition of H-2 or syngas decreases the emission indices of CO, unburned hydrocarbons, and organics. The higher is the H-2 amount in the blend, the more significant is the effect. The emission indices of NO2 and N2O, in this case, are essentially higher than that of NO. As N2O is a very harmful gas, its emission should be taken into consideration when evaluating the total emission of the engine at low load regimes.