We experimentally investigate the effect of thermal boundary conditions on the heat transfer characteristics in a turbulent boundary layer developing on a heated flat plate. The flat walls are divided into upstream and downstream portions and heated independently, thus yielding rapidly changing thermal boundary conditions in contrast to the uniformly heated case. We use laser Doppler anemometry and a response-compensated fine-wire thermocouple as a system for simultaneous measurement of velocity and temperature fluctuations. We measure turbulent heat fluxes and elucidate their characteristics based on a quadrant-splitting analysis and probability density functions. In the case of stepwise cooling, the wall-normal turbulent heat flux near the wall becomes opposite in sign to that in the uniformly heated case, whereas in the stepwise heating case, the turbulent heat flux becomes larger than that observed in the uniformly heated case. Both of the changes in the wall-normal turbulent heat fluxes are caused by the quasi-coherent motions of the same velocity field.