The displacement damage due to non_ionizing energy loss (NIEL) is the main reason of device-malfunction in spatial radiation environments. In the low energy range where the Coulombic interaction dominates, Mott-Rutherford differential cross section is usually used in its creatment. However, electrostatic screening of nuclear charges of interacting particles is not accounted for. The NIEL induced by low energy proton in Si and GaAs have been calculated using analytical method and Monte-Carlo code (SRIM). Thin_target approximation was used when calculating NIEL by SRIM code and the result compared with that of other authors' . The results show that thin_target approximation is reasonable and NIEL scaling is feasible. The NIEL values become lower after taking into account the screening effect. The results by SRIM code are 30% and 20% of Summers's results for Si and GaAs at 1 keV, respectively. The result is very important for spacecraft design.