We　observed　three　patterns　of　THz　absorption　spectra　alpha(nu)　of　crystals.　The　spectra　alpha(nu)　of　phonon　and　polariton　are　smooth　curves　for　perfect　single　crystal,　but　their　mathematic　expressions　are　different.　The　two　mathematical　expressions　are　simple　and　convenient　to　confirm　the　character　of　the　THz　absorption　of　a　crystal.　Some　protuberance　in　the　spectrum　alpha(nu)　is　from　some　defects　or　impurity　ion.