The instantaneous local heat flux was determined under knocking conditions in a square piston engine simulator to investigate the relationship for the phase and magnitude between the heat flux and the knock intensity. Two platinum thin film resistance thermometers recorded variations of the the cylinder wall surface temperature at different locations. The wall heat fluxes were then calculated from the measured temperature variations. Schlieren pictures of the combustion process showed the motion of the burned gas during knocking.Pressure oscillations of about 5 kHz occurred after autoignition. Oscillations of the heat flux were at the same frequency and in phase with the pressure oscillations. The maximum heat flux increased almost linearly with the maximum amplitude of the pressure oscillations for pressure oscillations greater than 0.5 MPa. The rate of increase is not uniform spatially. Near the knocking zone, the maximum wall heat flux is 20 MW/m2 for a pressure oscillation of 3 MPa. This is about five times larger than for the non-knocking case. The time-integrated heat transfer at the wall near the knocking zone also increases almost linearly with the amplitude of the pressure oscillations. The integrated heat transfer reaches a value approximately twice that of the non-knocking case. Heat fluxes away from the knocking zone were less than the values in the region near the knocking zone.