Abstract: The key techniques of quantitative estimating for metallographic grain size were discussed, the development situations of estimating for grain size at home and abroad were introduced subsequently, and the future of estimating for grain size was clarified in the end. For complex metallographic images, problems of accurate extraction and reconstruction of grain boundaries would be solved in the near future, and quantitative estimating for grain size by software automatically or with less human intervention is expected to realize correspondingly to meet with the actual needs in factory.