Butanol is an attractive alternative fuel by virtue of its renewable source and low sooting tendency. In this paper, three butanol isomers (n-butanol, isobutanol, and tert-butanol) were induced via port injection respectively and n-heptane was directly injected into the cylinder to investigate reactivity controlled compression ignition in a heavy-duty diesel engine. This work evaluates the potential of applying butanol as low reactivity fuel and the effects of reactivity gradient on combustion and emission characteristics. The experiments were performed from low load to medium-high load. Due to the different reactivities among the butanol isomers, the exhaust gas recirculation rate and the direct injection strategy were varied for a specific butanol isomer and testing load. Particularly, isobutanol/n-heptane can be operated with single direct injection and no exhaust gas recirculation up to medium load due to the high octane rating. As the load increases, all three butanol isomers displayed increased peak cylinder pressure and pressure rise rate. Especially, n-butanol cases yielded a pressure rise rate of 23.4 bar/oCA at medium-high load because of sub-optimal combustion phasing. It constrains the high load limit of n-butanol/n-heptane operation. While tert-butanol cases presented the slowest heat release rate and consequently the lowest pressure rise rates. Extremely low NOx emissions were achieved for all three isomers. Interestingly, tert-butanol/n-heptane operation stands out for showing ignorable engine-out soot mass in the whole testing range. N-butanol cases require the most direct fueling to phasing the combustion properly and displayed the highest soot mass and the highest number of particles in the accumulation mode. Among the isomers, tert-butanol/n-heptane operation achieved the highest gross indicated efficiency (above 52%) in most operating loads.