Compton suppression system was established to accurately and precisely measure low level radiation during in situ measurement of sites. HPGe detector was selected as the main detector and plastics scintillation detector was selected as the guard detector. Using 137Cs source, while changing the height(h) and width(w) about plastic scintillation detector the gamma spectrum was simulated using the Monte Carlo method. Also each was performance evaluated using reduction factor (RF). As a result the optimum result of height(h) 10cm x width(W) 10cm, 1cm forward disposition of plastic detector size was derived. The reduction factor (RF) according to optimum design condition was 2.97, and a maximum of 5.0 suppression effect was obtained near to the 420keV.