[Aim] To quantitatively evaluate the biological control effect of rust (Puccinia spegazzinii) in Mikania micrantha, an efficiently and accurate image processing technology was developed for calculating the relative lesion area (RLA) of M. micrantha leaf.[Method] In this study, we calculated the RLA of infected leaf in M. micrantha based on three methods, including image processing technology, grid method and photocopy weighing method, respectively. In addition, using the manual segmentation method to calculate the RLA as the standard, the absolute accuracy and absolute error of above three methods were calculated and used as the systematic and scientific evaluation index.[Result] The result shows that compared with grid method and copy method, the image processing technology based on Excess Green+Excess Red (ExG+ExR) algorithm segmentation disease spot can quickly and accurately calculate the RLA of infected leaf in M. micrantha with 98% absolute accuracy and 1.81% absolute error rate. In addition, it only takes 45.77s to process a 4608×3456 pixels color image.[Conclusion] Compared with the traditional method, because the image processing technology could accurately and quickly segment the lesion area and the healthy area, the accurate RLA of infected leaf could be obtained.