V-I transmission matrix formalism which is well known in microwave engineering is proposed to analyze the fiber Bragg gratings. The coupled-mode equation is derived mathematically from the transmission matrices,and the validity of V-I transmission matrix formalism is proved. Numerical simulations show that the V-I transmission matrix formalism is much more efficient than coupled-mode theory. At last,it is pointed out that the V-I transmission matrix formalism needs to be modified when it is applied to long-period fiber gratings.