We have studied the optical properties of three kinds of aromatic amino acids including tyrosine, tryptophane and phenylalanine using THz time domain spectroscopy between 0.2 and 1.6 THz.Comparing their absorption spectra,we find that tyros ine and tryptophane have sharp absorption peaks at 0.976THz and 1.465THz respect ively,while phenylalanine does not have absorption peak in this range.This resul t shows the capability of THz time domain spectroscopy to distinguish different molecules.Calculating by density functional,we found that the absorption peaks a re caused by the rotation and torsion of the molecules.The average refractive in dexes of these three amino acids are also obtained:they are 1.507,1.526 and 1.68 6 respectively.This work not only is significant for the research of molecular s tructure and molecular dynamics,but can also provide reference for studying othe r large biological molecules by THz time domain spectroscopy.