A number of studies in diesel dual fuel (DDF) operation which introduces natural gas from the intake pipe and ignites it by a diesel fuel injection in the combustion chamber have been conducted using conventional diesel engines. The present study investigated the influence of the ignition fuel on engine performance, combustion characteristics, and emissions with a combination of EGR and supercharging in DDF operation. The experiments employed iso-pentanol blended fuels for the ignition. Isopentanol is a next generation bio-alcohol fuel produced from cellulosic biomass, and actual use can be expected. The experiments were conducted at two CNG supply rates, 0% (ordinary diesel operation) and at a 40±4% (DDF operation) energy basis, and with EGR rates varied from 0 to 26%. The boost pressure was set at two conditions, 100 kPa (naturally aspirated, N/A) and 120 kPa (supercharged, S/C) with a supercharger. Four kinds of ignition fuels were used, JIS No.2 diesel fuel as a reference, neat methyl laurate (LME) which is a major component of coconut oil biodiesel, and two iso-pentanol blended fuels with 30% mass ratios, DiP30 (70% diesel and 30% iso-pentanol) and LiP30 (70% LME and 30% iso-pentanol). The results showed that regardless of the ignition fuel, the DDF operation with EGR and also supercharging resulted in substantial reductions in NOx and smoke emissions. However, the brake thermal efficiency decreased with increasing EGR rates, and with much longer ignition delays this tendency was more pronounced for the two iso-pentanol blends.