A control system of an internal combustion engine in which as fuel, a first fuel of ammonia and a second fuel easier to burn than ammonia are used. These two types of fuel are burned in the combustion chamber. A basic ammonia ratio is set in accordance with an engine load and engine speed. The set basic ammonia ratio is corrected based on at least one of a combustion state, knocking strength, temperature of an exhaust gas or temperature of a catalyst arranged in an engine exhaust passage, NOx concentration in the exhaust gas, actual compression ratio, air-fuel ratio, and fuel properties.