Ritonavir is an antiretroviral of the protease inhibitor class. It is used against HIV infections as a fixed–dose combination with another protease inhibitor, ritonavir (lopinavir/ritonavir). In the current research, the stimulated FT–IR biospectroscopy of liquid sample of Ritonavir was investigated. The stimulated FT–IR diffractions emitted through focusing the second harmonic laser beam Nd:YAG into the sample were recorded by Echelle spectrometer and ICCD detector. Increasing the energy of laser beam from 2.6 (mJ) to 16 (mJ) was led to increase in stimulated FT–IR signal but after breakdown threshold of liquid sample, more increasing of energy was led to decrease in stimulate FT–IR signals and for energies higher than 20 (mJ), they were disappeared. Ball–and–stick model of the ritonavir molecule, C37H48N6O5S2, as found in the crystal structure reported in Pharm. Res. (2001) 18 859–866 (CSD Entry: YIGPIO02). Colour code:   Carbon, C: grey   Hydrogen, H: white Nitrogen, N: blue   Oxygen, O: red   Sulfur, S: yellow Model manipulated and image generated in CCDC Mercury 3.8.