Experimental study on dual fuel HCCI-DI stratified combustion mode is investigated with port injection of n-heptane and direct injection of primary reference fuels (PRFs) with different octane numbers from 0 to 100. Firstly, in-cylinder stratifications of mixture concentration, composition, and temperature for this HCCI-DI combustion mode were numerically simulated. The effects of the octane number of the DI fuel, the overall equivalence ratio and the premixed fuel fraction by port injection on the combustion, emissions and performance characteristics are studied. Further, the premixed fuel fraction in this dual fuel HCCI-DI stratified combustion mode is optimized based on the octane rating of the DI fuels. It was found that at lower premixed ratio of HCCI-DI combustion, dual-fuel sequential combustion was observed when high octane number fuels were used as directly injected fuel. And the ignition of the direct injected fuels was mainly determined by the active radicals or thermal energy-active radical. At large premixed ratio, the ignition of direct injected fuels was determined by the thermal-atmosphere but almost not influenced by partial equivalence ratio of DI fuels and their octane number. (C) 2016 Elsevier Ltd. All rights reserved.