Photo-thermal effect is in general a well-established technique for remote and nondestructive material and structure evaluation. In particular, the method of transient thermal gratings is typically used for characterization of thermal properties and of low optical absorption in liquids. Conventional experimental configuration implies utilizing two mutually coherent recording beams to produce the thermal grating and one additional probe beam to detect it. We propose to perform recording and simultaneously probing the thermal grating using the same recording beams in configuration of dynamic two-wave-mixing (TWM), which simplifies the detection configuration and, as we hope, increases the sensitivity. In this configuration the sample is irradiated by the interference pattern of two coherent beams, in one of which the periodic phase modulation is additionally introduced. The output signal is detected as an amplitude modulation in one of the transmitted beams using conventional high-sensitivity lock-in amplification technique. The detected output signal is proportional to the amplitude of the thermal grating (but not to the grating diffraction efficiency as in coventional arrangement with additional probe beam), which also potentially increases the sensitivity. While the grating amplitude is evaluated directly from the output signal amplitude detected in this configuration, the photo-thermal grating formation time is obtained from position of the so-called "cut-off" frequency in the signal modulation frequency dependence. Details of experiments with this configuration using slightly dyed acetone sample at the wavelength 633 nm, which resulted in evaluation of the characteristic grating recording time and, finally, of the thermal diffusivity coefficient of the liquid, are presented.