The allocation of fixed transmission cost under hybrid transactions modes is investigated. Based on the algorithm of imputation of an impedance-branch dissipation power，an accurate analytical method for imputing power flow naturally and simultaneously to individual transaction is derived mathematically， where transaction power is the independent variable. Both the fixed cost and the imputation of branch power flow to transaction are analyzed to develop a concrete method for imputing fixed cost to individual transaction. The proposed method is applicable for electricity markets containing pool and bilateral transactions. It satisfies electric circuit laws and provides economic signals to users. Simulation results show that the proposed algorithm is effective and feasible.