Abstract: Reactivity control compression ignition (RCCI) has been shown to be effective in reducing nitrogen oxides (NOx) and particle matter (PM). A significant feature of RCCI is the ability to control the combustion process by creating reactivity stratification in the cylinder, which could reduce emissions. Since iso-butanol has a higher octane number, it can enhance the gradient of reactivity stratification, which will facilitate the control of combustion. The effect of iso-butanol on engine combustion and emission characteristics was investigated on a six-cylinder turbocharged diesel engine under RCCI combustion mode. In this study, iso-butanol (low reactivity fuel) was provided through the port injection (PI), and the diesel (high reactivity fuel) was injected directly into the cylinder. In the test, the combustion and emission characteristics of RCCI fueled with iso-butanol and diesel were investigated with different fuel supply strategies by changing the premix ratio (Rp) of iso-butanol and the start of injection (SOI) of diesel under low engine load. Four different premix ratios were selected, including 30%, 40%, 50% and 60%, and five different SOIs were designed, including -8°, -12°, -18°, -24° and -30° crank angle crank angle after top dead center(°CA ATDC). In this study, the engine was operated at a constant speed of 1 500 r/min, maintaining the brake mean effective pressure (BMEP) and the total cycle energy at 0.3 MPa and 1 280 J, respectively. The results showed that, with the advancement of SOI, the ignition delay (ID) was prolonged and the maximum heat release rate (HRRm) was increased first and then decreased. In terms of emission characteristics, with the advancement of SOI, the emission level of THC and CO decreased, and NOx emission increased first and then decreased. As SOI was further advanced to -30 °CA ATDC, it can be seen that the emission of CO increased when Rp was 30% or 40%. While the emission of PM at Rp from 30% to 50% showed a trend of decreasing first and then increasing, when Rp was 60%, it only showed a decreasing trend. In addition, the Rp of iso-butanol also had a large effect on RCCI combustion and emission characteristics. As the Rp increased, both the ID and CD (combustion duration) increased, while the HRRm decreased. In terms of emission characteristics, THC and CO increased and NOx decreased with the increase of Rp. When Rp was 30%, the total PM mass achieved the highest level under the four different Rp, and when Rp was 60%, it had a lower level of the total PM mass than that when Rp was 40% or 50% and SOI was -30 °CA ATDC. The size distribution of PM showed that the quantity of nuclear particle matter (PMN) was dominant, of which the diameter size was in the range of 3-30 nm. However, the proportion of PMN was very small for the total PM mass. In the mass concentration, the PM mass was dominated by the accumulated particle matter (PMA), of which the diameter size was in the range of 30-500 nm. The increase in the Rp reduced the whole PM quantity, but with the SOI advanced, the PMN quantity increased first and then decreased, while the PMA quantity decreased first and then increased, except when Rp was 60%. When Rp was 60%, PMA quantity only showed a decreasing trend with the advancement of SOI. The Rp had a complex effect on the mass distribution of PM. The most interesting part was that, when Rp was 60%, the total PM mass was higher than that when Rp was 40% or 50% at SOIs near the top dead center. The reason was that, when Rp was 60%, the peak value of PMA quantity was lower than that when Rp was 40% or 50% in the range of 30-500nm, whereas in the range 100-200 nm, more PMA quantity was exhibited, which lead to higher total PM mass. In addition, as the SOI advanced, the mass of PM generally showed a decreasing trend.