Electron flux distribution of a flat X-ray spectrum Very flat X-ray spectrum (gamma <= 2 below 80 keV) was detected at the burst maximum of 20 Aug 2002 flare. Fitting of an isothermal component plus a double power-law electron flux distribution function in the thick target model revealed the low energy cutoff of 80 keV which would mean a gap between energies of the thermal plasma (T ~ 10 MK) and the injected electrons. Assuming different shapes of electron flux distribution below the low energy cutoff we investigate their influence on the photon spectrum. The results are discussed in the frame of detectable changes in the RHESSI photon spectrum.