The model of two long-period gratings cascaded with different periods, lengths and gap is proposed. The transmission spectrum is analyzed by utilizing transfer matrix and coupled-mode theory. Theoretical analysis indicates the gap of cascaded long-period gratings (CLPG) has the least effect on the position of loss peaks. In certain wavelengths region the simulative spectrum is a straight line for particular structure of CLPG. The transparent phenomenom is observed and the condition of gratings transparency is obtained according to the model proposed. The transmissivity of CLPG is almost 0 dB as the periods and lengths of gratings are properly adjusted to match the transparent condition. The transparent phenomenom is verified by experiment.