Abstract: 【Objective】To establish the novel molecular quantitative assays for quantification of bacterial population of Xanthomonas oryzae pv. oryzae (Xoo) in the process of infection of rice.【Method】Real-time quantitative polymerase chain reaction (RTQ-PCR) assays based on SYBR Green I technology have been developed to target lipA and purH for the quantification of in planta growth of Xoo.【Result】The changes in bacterial population density in planta measured by RTQ-PCR assay is similar to those assessed by bacterial plate counting. There is no discernible difference between the two primer sets evaluated for RTQ-PCR. Bacterial accumulation within rice showing no disease symptoms was observed at 3 d post-inoculation (dpi), then the bacterial density within rice increased significantly 5 dpi with rising bacterial leaf blight, and bacterial numbers reached a peak and maintained a high population 9-14 dpi when the plants displayed severe disease symptoms. Such a relation between bacterial population density in planta and host plant disease progression might be associated with quorum sensing of the pathogen.【Conclusion】The results of this study illustrate that RTQ-PCR can be successfully used to directly and accurately quantify Xoo within leaf tissues of rice. Furthermore, “bacterial target gene copies - total DNA amount - bacterial population - host disease progression”, a hypothetical model of the pathogen assessment, has been proposed for accurate monitoring of bacterial infection process of rice by Xoo, which might be applicable to the molecular quantification of other bacterial and fungal diseases of rice.