Key problem of sub-millimeter waves or terahertz system is the very small output power of the source. Power combining technology is a suitable approach to increase the output power. However the conventional power combining methods used in microwave and low millimeter wave bands such as circuits combining or waveguide spatial combining all suffer from the higher loss and difficult for manufacture and so on at sub-millimeter waves. In this paper an approach of beam power combining based on quasi-optical technology is proposed. It has advantages of low loss, high combining efficiency and relative easy for manufacture. The analysis, simulation and experiments results are presented. High combining efficiency has been obtained.