Potential clinical use of ultrasound in enhancing the effects of anticancer drugs in the treatment of cancers has been highlighted in previous reports. Increased uptake of drugs by the cancer cells due to ultrasound has been suggested as a mechanism. However, the precise mechanism of the enhancement has not yet been elucidated. Here, the combined effects of low-intensity pulsed ultrasound and doxorubicin (DOX) on killing (loss of viability and apoptosis) of U937 cells, and the involved mechanism were investigated. The synergistic enhancement in cell killing and additive enhancement in induction of apoptosis were observed above 0.3W/cm^2. No enhancement was observed at intensity of 0.2W/cm^2 in cell killing and induction of apoptosis. The mechanisms of the enhancement are 1) Increase in incorporation of the DOX by ultrasound involved with sonoporation. 2) Enhancement of the cavitation by DOX. 3) Cavitation is required for the enhancement of the DOX effects.