Abstract: In model teats of hydrodynamic torque converter and hydrodynamic coupling,the condition in which the Reynolds number la oqimlj Is often difficult to satisfy. For predicting the efficiency in the case that the Reynolds number is different, some formulas have already been presented by some investigations. It seems, however,the calculating results of tlioso formulas nro always reliable as thay are bused on the assumptiong that nil hydraulic losses are friction losses. In this paperThe author presents u new method to predict the offlclency of prototype hydrodynumlc torque convortor and hydrodynnmle coupling uslng; the model teat detn tit two different speed of tho model impeller. Test results show that this method can load to satisfactory results,, In addition some factors which nro of Importance to predict tho efficiency of prototype aro also dlacussed.