The size of fuel propulsive droplets is one of the effective parameter in improvement of the mixture of air and fuel as well as combustion. The effects of Pressure on the average diameter of fuel propulsive droplets sizing and effective mass fraction in a duct are numerically investigated in the present paper. We named the mass of fuel vapor inside the flammability limit as the efficient mass fraction. The inlet pressure of entrance airflow is varied as 1, 2, 3, 4 and 5 (atm) to examine its effects on the fuel droplets and fuel/air mixing phenomena. As well, by growing the entrance air flow velocity from 36 to 50 (m/s) we have repeated this test again, which provides worthwhile information for the combustor design work. To fulfill the calculations a modified version of KIVA-3V code which is a transient, three-dimensional, multiphase, multicomponent code for the analysis of chemically reacting flows with sprays, is used.