We performed the study of a solar flare of GOES class X1.0 which occurred on 2002 August 21, peak time 05:34 UT. A new method was applied for temperature determination of the plasma seen in TRACE images. This method makes use of the detailed knowledge of the TRACE point spread function (PSF) function. Substantial dependence of TRACE PSF on wavelength allows for determination of temperature from a single TRACE image even in these portions of the image which are saturated. We also determined the average temperature of this flare from the soft X-ray spectra measured by RESIK Bragg spectrometer aboard CORONAS-F satellite. The results obtained from TARCE and RESIK data are compared.