Abstract Monte Carlo method has been used to evaluate the X-ray intensity distribution funetion in freestanding thin films and in surface layer of same thickness of the same material. A simple equation of the X-ray emitting intensity from a thin film on substrate has been proposed to correct the influence of Z. A. P. on the determination of film thickness. The accuracy of the present measurement is higher than that of methods previously. developed. Experimental results are presented for Ta2O5 film on GaAs and for ZrO2 on Si. They are in agreement with those obtained by ellipsometry.