The moisture of the sand in concrete industry is important for concrete quality. There are some different methods to measure the moisture in the sand. The microwave sensors are commonly used. The mix design or batch size frequently changes in some case. The gray correlation theory of gray system theory was applied in slump evaluation where the gray correlation coefficient and gray distance measurement were made use of in calculation estimating slump or the moisture of the sand. Then, the gray confidence interval of estimated value of moisture of the sand was calculated based on gray confidence level.