An oligonucleotide primer pair was designed and synthesized after comparison and homological analysis of rDNA ITS sequences among Phytophthora sojae, its related Phytophthora species，and allied fungal and bacterial species from GenBank. PCR amplifications were carried out for 140 isolates including Phytophthora sojae.It showed that only isolates of Phytophthora sojae can be amplified and a special fragment of 288bp were produced by the primers. These primers were used to detect Phytophthora sojae in pure culture，inoculated diseased soybean plants， and inoculated soil samples. The detection protocol has good sensitivity to diseased tissues.