Six types of mixed cereals powder were prepared using different mixing ratios of barley, proso millet, sorghum, and adlay, and their nutritional components and functionalities were analyzed. The mixed cereals powders contained high levels of crude protein. The free amino acid and mineral contents of Sample E were the highest among all the samples. Fructose, glucose, sucrose, and maltose were detected as the four major free sugars in all the mixed cereals powders, and Sample D had the highest amount of reducing sugar among all the samples. The total polyphenol and total flavonoid contents of Sample F were significantly higher than those of the other samples. Sample C showed the highest level of DPPH radical scavenging activity and reducing power. As for the ABTS radical scavenging activity, the IC50 of Sample D was the lowest among all the samples, but did not significantly differ from that of Sample C. Therefore, we suggest 2:2:1:1 (in Sample D) as the best mixing ratio of barley, proso millet, sorghum, and adlay to produce a mixed cereals powder with excellent nutritional value and antioxidant activities.