This paper proposes some new ideas on the theory of coupled waveguides which was formerly discussed in literature 1. A new suggestion on inter-boundary condition, and a new method of treatment have been offered. Two forms of coupled slots, shallow and deep, are considered. The electric field in the mouth of slot is estimated by approximate methods, then the electromagnetic field both in coupled rectangular and circular waveguides with their tensor Green functions is derived. It has been pointed out in this paper that the results of application of boundary condition, the characteristic equation, can be accounted for as the equality of admittance of two LC circuits. There are some discussions on the relations between propagation constants of coupled waveguides and those of individual waveguides. As concluding remarks, some simple applications of the theory are mentioned.