This paper presents an optimal design procedure of an ammonia synthesis reactor using genetic algorithms (GAs)-search and optimization techniques based on principles of natural genetics. GAs are chosen as an optimization tool simply because of their successful application to many engineering optimization problems. The optimal problem requires maximisation of an objective function subject to a number of equality constraints involving solution of coupled differential equations. Although there exist at least a couple of other studies on the optimal design of ammonia synthesis reactor, they have ignored some terms in the formulation of the objective function, for which the reported optimal solution does not match with the solution obtained using an enumerative search technique. This paper includes those terms and applies GAs to find the optimal reactor length. Simulation results with GAs find optimal reactor lengths at various feed gas temperatures at the top of the reactor, The results of this paper are also found to agree with the reactors used in industries. The successful application of GAs in ammonia reactor design suggests GAs immediate application to other reactor designs or modelling.