Harmonic image (HI) has been proposed to be promising for visualizing lesions produced by therapeutic high intensity focused ultrasound (HIFU). The study characterizes harmonics generated from the bubble cavitating at the focal region of a therapeutic HIFU field in response to a typical diagnostic ultrasound. Based on Gilmore model, it was simulated the nonlinear dynamics of the bubble being resonated at 1 MHz of the therapeutic ultrasound and driven by a typical 3.5 MHz diagnostic pulse. It was shown that harmonic generation increased with MI in a sigmoid pattern where the rapid and transient changes occurred between 0.5 and 2 in MI. For whole ranges of MI (less than 8), the sub-harmonic was the predominant in magnitudes over other harmonic bands. This reveals that, if HI is considered for improving the detection of focal legion highly cavitating caused by a HIFU field, the sub-harmonic component would be a preferred parameter rather than the 2nd harmonic which has been commonly used in current harmonic imaging.