Abstract: A pedestrian portal monitor for special nuclear material (SNM) and its software are developed. Its performances are as follows:detection sensitivities are 1.03 g and 0.031 8 g respectively for high enrichment uranium and low burned plutonium; false alarm probability is 1/5 500. The portal monitor for SNM is used to prevent unauthorized removal of fissile materials such as 235 U and 239 Pu in protected area. The monitor uses a personal computer to provide automatic operation. The sequential probability ratio test(SPRT) algorithm is used to determine the presence of SNM over background.