“Diesel knock” is associated with extremely high in-cylinder pressure rise-rates, strong pressure oscillations, destructive engine vibration, as well as audible noise. It not only exists in traditional Diesel combustion, but also is a main concern for premixed charge compression ignition (PCCI) combustion. Recent work showed that during Diesel knock the flame’s motion synchronizes with the in-cylinder pressure ringing. To improve the optical method and investigate this relation further, we imaged the flame luminosity in an optically accessible engine during Diesel knock at very high frame rates (60 kHz). First, the optical knocking time interval was determined based on the temporal variation of the mean image intensity. Within this time interval, the instantaneous flow fields were calculated by “optical flow” based on cross correlation. From these time-series velocity vectors, the oscillation frequencies were obtained and compared to those from pressure-trace analysis and theoretical calculation. The images show a “sloshing” motion of the flame, with nearly the same frequency content as that of pressure ringing. The velocity of the flame movement is found to be subsonic, while the temporal variation of the velocity field suggests the pressure wave is a supersonic shock wave. As opposed to pressure-trace analysis, the optical diagnostic is spatially resolved. That means the result is independent from the cyclically varying position of the oscillation’s nodal lines, containing all the oscillation information in a single cycle, including oscillation direction. The flame luminosity images indicate that the knock intensity is dependent on the spatial distribution of the auto-ignition sites at the start of combustion.