The strange properties of Left - Handed Materials (LHM), whose permittivity and permeability are both negative, is briefly introduced. The electric- and magnetic-field components are given in rectangular waveguide loaded with LHM by use of field component matched method. According to the obtained dispersion equation, the tendency of cutoff wavelength is analyzed where the dielectric constant and the slab thickness are different. And the β − − ω diagrams have been obtained for various permittivity’ constant. The propagation characteristic of the LHM loaded waveguide shows very different ones compared with the normal waveguide.