In order to meet the requirement on the stringent emission regulations recently, more and more research works have been focused on homogeneous charge compression ignition (HCCI) and partially premixed combustion (PPC) or partially premixed compression ignition (PCCI) as they have the potentials to produce low NOx and soot emissions without adverse effects on engine efficiency. The mixture formation and stratification charge largely influence the combustion behavior and emissions for PPC/PCCI. An ultra-high speed burst mode laser is used to capture the mixture formation process from the start of injection until several CADs after start of combustion in a single cycle. To the authors’ best knowledge, this is the first time that such a high temporal resolution, i.e. 0.2 CAD, PLIF could be accomplished for imaging of the in-cylinder mixing process. The capability of resolving single cycles allows for the influence of cycle-to-cycle variations to be eliminated. This ability to study individual cycles aids the understanding of the mixture formation process as well as the cycle-to-cycle variations. Strong air entrainment at the boundary layer can be clearly observed and followed as the mixing process progress. The formation of eddies created by the shear force and their rotational motion can be continuously observed during the mixing process. The interaction between two adjacent spray plumes in the recirculation zone is well captured and studied. Moreover, through high-speed chemiluminescence imaging the auto-ignition position and early flame development can be studied together with the fuel distribution in the combustion chamber.