Stratified charge compression ignition (SCCI) combustion was investigated on a single-cylinder, four-stroke diesel engine with n-heptane. A premixed atmosphere was realized by the early injection at the intake port, while the rest of fuel was directly injected into the cylinder. Premixed ratio (defined as the ratio of fuel amount in port injection divided by the total fuel amount in the charge) and fuel supply timing of direct injection were altered to explore their influences on SCCI combustion, emission, and engine performance characteristics. The results show that increased premixed ratio and earlier fuel supply timing can remarkably enhance the peak cylinder temperature and pressure while shortening the main combustion duration. NOx is strongly dependent on fuel supply timing, whereas premixed ratio and equivalence ratio play a secondary role. CO can be effectively reduced by increasing premixed ratio and advancing fuel supply timing. HC rises with increasing premixed ratio but is insensitive to the change of fuel supply timing. Finally, indicated thermal efficiency (ITE) drops with retarded fuel supply timing, and the maximum indicated mean effective pressure (IMEP) can be achieved by optimizing the premixed ratio and fuel supply timing.