During the treatment of metal with concentrated heal sources (plasma and laser welding, cutting etc.), high temperature gradients are created within the liquid metal pool. A capillary wave propagating along this pool carries non-uniformly heated metal out to the surface. Since the surface tension coefficient sigma depends on temperature, surface forces arise, which are directed from these spots with low surface tension toward the spots with high sigma. These forces can amplify the capillary wave, thus leading to instability. A simple expression for the increment of this instability for the molten pool of finite depth is obtained. It is shown that, for conditions of plasma are welding, a wide spectrum of capillary waves exists in a liquid metal pool. The analysis of the non-linear phase of the instability shows the saturation of the waves amplitude. The influence of the waves on the thermal conductivity and on the viscosity of the metal in the pool is estimated. It is shown that, for high energy flux densities, capillary waves might increase the effective thermal conductivity several times. Their influence upon the viscosity is even more pronounced. These estimations qualitatively agree with some recent data on heat and mass transfer intensity in the liquid metal during welding.