Transmission cost allocation methods have been varied depending on local context of electricity supply industry. There is a common principle that transmission line capacity should be properly allocated to accommodate actual power delivery with adequate reliability margin. This paper, therefore, proposes a method that allocates transmission embedded cost to both generators and loads in an equitable manner, incorporating probability indices to allocate transmission reliability margin among users in both supply and demand sides. Probabilistic indices so called TIRM and TERM decomposed from TRM are introduced, making true cost of using overall transmission facilities. A numerical example on a simple six-bus system with multiple-circuit transmission lines which represent a characteristic of practical system is also presented to illustrate the application of the proposed method.