The performance of a hydrofoil sailing boat, which has dihedral-angled and surface-piercing foils, at high speed is described. A simple method of performance prediction was devised, which considered two important factors for sailing boats as follows : (i) variation of hydrodynamic drag caused by change of heel and leeway angles ; (ii) limitation of sailing performance due to capsizing. For the present study one typical sailing hydrofoil configuration was used, i. e. a 5. 08m long catamaran, with a pair of surface-piercing front (main) foils having 40 degree dihedral angles, and one horizontal rear foil, which formed an inverted “T” in combination with the rudder. The calculations aimed to predict the effect of dihedral angles, width between the foils, and weight of the boat on performance. The results obtained were as follows : (1) the present method gives good estimated values for the performance of a hydrofoil sailing boat with dihedral-angled and surface-piercing foils ; (2) the sails should be trimmed to produce the maximum thrust force, but the performance of a boat to a beam reach is subject to limitation due to capsizing, and to windward, due to leeway angle ; (3) although the true wind angle varies extensively, the apparent wind angle at high speed varies much less, so that the aerodynamic drag angle of the sail can be regarded approximately constant; (4) the predicted heel angle is between 0 and-6 degrees at high speed (heel to weather-side); (5) the aerodynamic force coefficients of the sail estimated from the seatrial are smaller than the wind tunnel test data ; (6) there are small differences between the predicted and measured immersion of the hydrofoils ; (7) the weight of a boat has an appreciable effect on the light wind performance ; (8) the dihedral angles and width between the foils have a considerable effect of the boat performance. The optimum dihedral angle is between 40 and 45 degrees, and the optimum width between foils is around 1. 21. 6 h_s , h_s beeing the height of C. E., the centre of sail force, above the foil bottom.