The near-field heterodyne grating (NF-HDG) method was applied to flow rate measurements in a microtube. The NF-HDG method is a newly developed optical technique based on photothermal effects. In this technique, pump light is shone on a transmission grating in front of a fluid channel and the inside liquid is heated with a pattern of the grating due to the Talbot effect. Another probe light is similarly shone on the same place as the pump light, and the diffraction by the transmission grating (reference) and the diffraction by the temporally generated thermal grating inside the fluid channel (signal) are mixed and detected (heterodyne detection). Theoretical analysis reveals that the dependence of the heterodyne signal intensity on the flow rate originates from the change in the heterodyne phase difference between the signal and reference, and the experimentally obtained calibration curves can be fitted with the theoretically predicted function. Furthermore, the optical setup was optimized based on the theoretical analysis of the Talbot effect. Flow rates of the order of nl/min were quantitatively measured, and the detection limit of the flow rate was 17 nlmin. © 2007 American Institute of Physics.