Mean runoff ratio (fR) is defined as a ratio dividing runoff height (RF) by annual precipitation (P), namely fR=RF/P. Relation between annual precipitation and runoff ratio is expressed by three ways ; fR=a0 (constant), fR=a1P and fR=a2Pn (n<1). The case of constant fR includes an essential consistency that the runoff is generated even in very small amount of P. The latter two cases are not unreasonable, and the justification of them are discussed in this paper. Runoff height for the latter ones are obtained by RF=a1P2 and RF=a2P(n+1). Runoff ratios for the Tama River, the Sumiyo River and data from Selles’ textbook (1965) are summarized. Relation between annual precipitation and runoff height deduced from Kayane and Takeuchi (1971), Otsuki (1986) and Nakano (1974) are also summarized. These data suggest that runoff height may have the form of RF=a2(P-Em)(n+1), where Em is the minimum evapotranspiration. This means that fR has the form of (P-Em)n, and fitted curve for river basins in Japan is fR=0.0048 (P-500)O.67, for Budyko’s data is fR=0.017(P-200)0.5.
