The rate of combustion of liquid fuel spray injected into the combustion chambers of diesel or gas turbine engines is calculated in the following way : (1) As the distribution functions for the number and the weight of the sprayed drops are expressed generally by the equations (7) and (10), the experimental indices α and β have been determined for various atomizers and found to be α=-0.5∼2 and β=1. (2) Assuming that combustion rate of the sprayed drops obeys the same law of the combustion of a single fuel droplet expressed in equation (11) and considering that the number of sprayed drops remains constant during the course of combustion, the equation for the weight distribution of the sprayed drops are derived as (17) and (20), the graphical integration of which, up to the maximum diameters, gives the total amount of drops unburned. (3) Thus the amount of burned drops w_b are given as a function of time t and the distribution indices α and β the approximate formula for which is given as (24).