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Hot-pressing powdered magnesium fluoride, under the conditions of an air, a temperature varying within the range from 510 to 690°C, a pressure of 40,000 psi, and a holding time under pressure of 30 minutes, resulted in a polycrystalline plate or shape with high in-line optical transmittance. The effect of grain growth of the sample on the optical transmittance in the window region is discussed. The positions of the transmittance cutoff for long wavelength and short wavelength are investigated. It is proposed that the shift of short-wavelength transmittance cutoff toward longer wavelength is caused primarily by the pore growth that is associated indirectly with the grain growth, that the shift of the long wavelength transmittance cutoff is almost independent of the microstructure, and that the shifts of both long wavelength and short wavelength are very dependent on the sample thickness. The importance in optical transmittance of the anisotropic grain compared with the pore might become significant when the thickness of a pair of samples is considerably reduced.