A variety of applications are envisaged in which image analysis based on digital image processing will play an increasingly important role. Microstructure impacts the mechanical properties of metals, alloys, and composites. Therefore the microstructural study is critical. In this research, microstructure analyses were conducted using an image processing technique through Image J software. Image processing is a tool that allows for information to be acquired quickly, fully, and accurately. Pre-processing of microstructure images is the process of enhancing the fine microstructure before quantitative analysis. This paper focused on analyzing microstructure grain boundary and grain size in AZ 61 alloy. In this case, the digital image is transformed into a binary image having been segmented at a particular threshold value. This way we can alter the intensity of the image to more accurately study the characteristics of microstructure and grain size. A new microstructure image smoothed grain areas and accurate grain boundaries were identified. The microstructure image analysis results provide us with a clue to the explanation of the strength mechanism as those observed in the secondary phase of AZ61 alloy.