Thermodynamic parameters are fundamental quantities to characterize chemical reactions. For a long time, these parameters have been measured only under equilibrium conditions. However, our group has developed a novel time-resolved detection method of the thermodynamic parameters using the pulsed laser induced transient grating (TG) technique. This technique has been applied to various photoreceptor proteins and succeeded in determining thermodynamic parameters of transient intermediates during protein reactions, such as enthalpy change, volume change, heat capacity change, and thermal expansion coefficient change. Recently we have developed an instrument to introduce the pressure to this powerful method and expanded its capability toward the time-resolved detection of the ‘isothermal compressibility (β_T)’, which directly reflects the structural fluctuation and hence is important in the protein science. The fluctuation of a protein is considered to play an important role for its function and attracts much attention. Hence, β_T of reaction intermediates can offer the direct knowledge how the fluctuation is involved in protein reactions. In this article, we briefly review the pressure variable TG method including its principle, a specially designed optical high-pressure cell, and our recent results on the first detection of β_T for intermediates of the protein reaction.