We propose a thermally actuated tunable grating for measuring the beam profile of a {\text{CO}}_2 laser. The grooves of a transmissive grating are filled with a liquid whose refractive index depends on temperature. A visible laser as a probe and a {\text{CO}}_2 laser as a heat source are illuminated on the grating. The {\text{CO}}_2 laser is absorbed, and depending on its beam profile, a temperature profile is induced on the grating. The refractive index of the heated liquid is changed, resulting in a change of efficiency of the grating for the probe laser. By using the 1st orders of diffraction in a $4f$ imaging system, the beam profile of the {\text{CO}}_2 laser is imaged onto a CCD camera by the probe laser.