A model of Waveguide Photonic Band Gap (WPBG) structures, based on Leaky Mode Propagation (LMP) method, has been developed to perform a complete analysis of the electromagnetic (e.m.) wave propagation including the radiation loss for a structure of finite extension. The model allows to determine the mode propagation constants, electromagnetic harmonics and total field distribution, transmission and reflection coefficients, total forward and backward power flow in the structure, guided power and total losses. Moreover, the mode amplitude attenuation constant, radiated power, the angle of radiation and radiation efficiency both in the cover and substrate, for both TE and TM modes, can be determined.