AbstractImprovement of fusion reactivity (〈σv〉) and fusion power multiplication factor (Q) in the presence of fast ions has been investigated with the analytical solution of the velocity distribution function. Fast ions created by only NBI heating, only ICRF heating and a combination of NBI and ICRF heating are considered in a D-T plasma. The dependences of 〈σv〉 on the plasma temperature, the deposited power density, the electron density and the injected beam energy are clarified. Calculations of evaluating Q employ a simple plasma model in which the radial profiles of the plasma temperature and density are the same as those observed in recent experiments. 〈σv〉 and Q are enhanced by only NBI heating, the combined NBI and ICRF heating and only ICRF heating in this order under the same heating power. However, the increase in 〈σv〉 and Q for the former two methods is almost the same. In the presence of fast ions, the enhancement of 〈σv〉 and Q is larger than the increase in the plasma pressure.