The effective combination of microbubbles and ultrasound in HIFU therapy has been investigated through a numerical simulation. The equations for conservation of mass, momentum and energy are solved for a microbubble-infused mixture, and model HIFU therapy in the presence of microbubbles. The bubbles follow Rayleigh-Plesset equation, and are coupled with the mixture using the Euler-Lagrange method. The impacts of both the volume fraction due to the microbubbles and the amplitude of the ultrasound on the heat distribution are analyzed by using a two dimensional cylindrical simulation of the propagation of focused ultrasound (1MHz) through a mixture of bubbles 1.3mm in diameter. As the initial void fraction increased, the shape of the high temperature region was observed to change from cigar shape, to tadpole shaped, to cone shape. It was numerically confirmed that the temperature dramatically increased by means of inertial cavitation. The results show that the effectiveness of HIFU therapy increases with increased initial void fraction, as well as an input ultrasound amplitude above the threshold required for inertial cavitation.