The neutron diffraction pattern of powdered KCl has been measured using the triple-axis spectrometer both in the double-axis and the triple-axis modes. The thermal diffuse scattering (TDS) correction, which is very important in the double-axis pattern, is essentially eliminated in the triple-axis mode. Theoretical calculations are presented which give the relative TDS correction for the two cases by calculating the energy distribution of TDS. The Debye-Waller coefficient B using the triple-axis data is found to be (2.17 ± 0.16 A2), which is in agreement with the TDS-corrected values of other workers.