The present study is aimed to develop an effective method for the detection of Liberobacter asiaticum that causes citrus Huanglongbing disease. It was shown that the DNA of the pathogen could be amplified by polymerase chain reaction (PCR) in vitro and the appearance of such DNA was significantly detected in the samples of infected citrus trees of the various cultivars. To use PCR for the detection of the infected trees without any symptom yet at the early stage of the infection. This technique can be effectively applied to screen disease free seedlings, establish disease free citrus nurseries, and control the spread of the diseased seedlings.