In microwave rotational spectroscopy, absorption spectra arise from molecular rotation and correspond to transitions between the rotational energy levels associated with a given vibrational state of a particular electronic state. The rotational transitions, which fall in the microwave region, are induced through the interaction of the molecular electric dipole with the electric vector of the radiation. The microwave region extends roughly from〖10〗^2 μm to 〖10〗^4 μm. The article explains the pure rotational absorption spectra of diatomic molecules such as HCl, HBr, HI, CO, … etc. In order to explain the absorption spectra, diatomic molecules are treated as rigid rotor and non-rigid rotor.