A digital computer program was used to predict theoretical performance for ammonia as a fuel for Diesel engines. The theoretical model employed is the Diesel cycle, and products of combustion were treated as a chemical equilibrium system. Results are presented for ammonia and in addition for cetene, a common hydrocarbon fuel. It is thus possible to directly compare the performance predicted for ammonia to that predicted for commonly employed hydrocarbon fuels. The following conclusions regarding performance of ammonia are based upon this work: (1) In the event of developments leading to successful combustion of ammonia in Diesel engine application, ammonia would be expected to yield up to 10% more power output than typical hydrocarbon fuels operating under the same conditions. (2) The thermal efficiency obtained with ammonia would be expected to be greater than that for hydrocarbon fuels by as much as 20%. (3) Specific fuel consumption for ammonia may be expected to be approximately 2-1/2 times greater for ammonia than for hydrocarbon fuels. (4) The exhaust concentration of nitric oxide resulting from the use of ammonia should be substantially less than that produced by hydrocarbon fuels.