A molecular beacon probe was used in the detection of prawn white spot bacilliform virus (WSBV). The probe possesses a stem and loop structure. The sequence of loop is complementary to the WSBV DNA, and the stem is formed by two complementary arm sequences which are unrelated to the WSBV DNA. The probe underwent a fluorogenic conformational change as it hybridized to the WSBV DNA. The probe could not affect the amplification , and was specific and sensitive when used in PCR. It was showed that the fluorescent intensities increased as the cycles of PCR and the number of target DNA copy increased.