A calculation method for fusion product momentum deposition in denseplasma spheres has been developed and applied to laser imploded DT targets. The net radialmomentum deposition from αparticles accelerates the expansion of the plasma sphere and thusreduces the rate of thermonuclear reactions. However, this effect is not so significant aspreviously expected because the rate of momentum deposition by the α particles is much lessthan the local pressure gradient around the burn front. The momentum deposition fromneutrons is negligibly small because of its long mean free path.