Cavitation bubbles are known to enhance HIFU treatment. To generate them at a high efficiency, ultrasound waveforms emphasizing either positive or negative pressure were synthesized by superimposing the second harmonic onto the fundamental and combined in an exposure sequence. In this study, four different types of sequences were compared. Negative-pressure emphasized waves immediately followed by positive-pressure emphasized waves resulted in the highest efficiency. The pressure-release surfaces provided by the cavitation bubbles generated by the former waves may have converted the latter waves to highly negative-pressure emphasized waves.