Lesion area estimation of plant leaf is a common method of investigating filed plant diseases.However,the conventional artificial estimation method,which depends on human eye identification,has a series of questions such as inaccuracy of plant disease rating,heavy workload of investigator,variation of scale from one investigator to another and so on.It will improve the accuracy of computation and plant disease rating to apply digital image recognition technology in calculating leaf lesion area.With the smart phone based on Windows Mobile system as development platform,this paper develops an image analyzer for field plant disease by employing C# programming technology.The analyzer collects leaf image of field plant by the photo function of smart phone.It can also analyze the percentage of lesion area to obtain the severity of plant diseases.The simulation test shows that this analyzer operates siThe lesion area estimation of plant leaf is a common method of investigating filed plant diseases.However,the conventional artificial estimation method,which depends on human eye identification,has a series of questions such as inaccuracy of plant disease rating,heavy workload of investigator,and variation of scale from one investigator to another.It will improve the accuracy of computation and plant disease rating to apply the digital image recognition technology in calculating the leaf lesion area.With the smart phone based on the Windows Mobile system as the development platform,this paper develops an image analyzer for field plant disease by employing the C# programming technology.The analyzer collects leaf images of field plants by the photo function of smart phones.It can also analyze the percentage of lesion area to obtain the severity of plant diseases.The simulation test shows that this analyzer has the advantages of easy operation and accurate analysis.Furthermore,the analysis functions can be expanded by software development.