A new technique to obtain viscoelastic property of lubricants under high pressure over 2 GPa is proposed. Tested lubricants are BCCM and DMP, which have high viscosity-pressure coefficients. From the difference of the ruby fluorescence wavelength between (0001) and 112^-0) ruby plate in diamond-anvil apparatus, shear stress relaxation can be observed. By applying Maxwell model for the relaxation analysis, the extremely high viscosity (over 1015Pa·s) is obtained and is almost fitted with the extrapolated value of the existed one. Bulk viscosity and elastic modulus are also obtained and are consistent with the existing values.