This Bureau of Mines report describes the method of obtaining a strength index for rock drillability, particularly for percussion drills. The original protodyakonov method has been slightly modified, essentially to eliminate uncertainties in the measurement of the volume of fines and to avoid conjecture in selecting the representative value as the strength coefficient. A theoretical consideration of the relevant parameters in the method was extended from a comminution theory; hence, the meaningful value was obtained experimentally by plotting the ratio of the number of drops and the volume of fines against the number of drops. The minimum ratio of the number of drops to the volume of fines in the plot was selected as the coefficient of rock strength. The modified method of obtaining the strength coefficient preserves the original protodyakonov test apparatus and the merit of simplicity of the test procedure. The modified coefficient of rock strength shows its usefulness in predicting penetration rates with greater reliability than other mechanical properties. Consequently, the drillability curves were drawn using the modified coefficient of rock strength as the parameter for penetration rate prediction in the field.