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esented that the ultimate shear strength of beams without shear reinforcement strongly depends on their size. Until now, however, little research on the size effects of high-strength concrete deep beams was available. Accordingly, the objective of this study is to grasp the size effects of high-strength concrete deep beams. This experimental program was intended to understand the influence of the concrete strength and shear span-to-overall depth ratio(a/h) on the size effects of reinforced concrete deep beams without shear reinforcement. Twenty two simple beams were tested under two point symmetric top loading. From the analysis and investigation of this experimental results, it is concluded that the size effects on the ultimate shear strength of deep beams are hardly influenced by the concrete strength, but it seems relatively dependent on a/h. Also, it is investigated that the ACI and AIJ specifications related with the ultimate shear strength of deep beams, which were mainly specified by the experimental results of slender concrete beams, tend to overestimate with the increment of overall depth(h) under the equivalent a/h, because they do not reflect the size effects.