The dispersion relation of spin wave in Fe3Si at 14 K has been measured by inelastic coherent neutron scattering. The spin wave stiffness constant D0, which is obtained experimentally from quadratic dispersion of spin wave, equals 270 meV ?2. A calculation of effective exchange integral Jeff has been done in the frame of Heisenberg model. From the temperature dependence of magnetization, a spin wave stiffness constant Dm(0) can be derived also. There is an indication of the existence of Stoner excitations from a comparison of D0 and Dm(0). The temperature dependence of spin wave stiffness constant is also gaven.