The effect of a diabatic forcing on downslope windstorms is analytically investigated in a 2-dimensional steady hydrostatic system. The environmental flow is assumed to have a uniform horizontal velocity with a constant buoyancy frequency. A diabatic forcing is considered in which the material derivative of potential temperature is proportional to vertical velocity in each region of positive or negative vertical velocity. The proportionality constant for negative vertical velocity is not greater than for positive vertical velocity, and may be zero. The result shows that the downslope wind speed is diminished by the diabatic forcing. The downslope wind speed is smaller than the adiabatic counterpart, and greater than that in the reversible case (i. e., the case of the same proportionality constant for the negative vertical velocity as for the positive vertical velocity).