The infrared spectra of YBa 2Cu 3O 6.3 and PrBa 2Cu 3O 6.3 ceramics and films with c axis orientation were reported. In PrBa 2Cu 3O 6.3 , 9 phonon modes corresponding to 5E u+4A 2u vibration were observed, where the E u and A 2u modes of Pr element were found at 170 and 198cm -1 , respectively.In YBa 2Cu 3O 6.3 ,10 modes corresponding to 6E u+4A 2u vibration were observed, where the E u and A 2u modes of Y element were located at 191 and 217cm -1 , respectively. Since YBa 2Cu 3O 6.3 and PrBa 2Cu 3O 6.3 possess similar crystal structures, by comparing and analyzing their E u and A 2u modes based on the regulation that the vibration frequency is inversely proportional to the square root of the mass, it can be inferred that the valence of Pr in oxygen deficient tetragonal PrBa 2Cu 3O 6.3 is 3 +.