A mathematical model is developed to study the transient behavior of a two stroke or four stroke, single cylinder I.C. engine during cranking and starting by a starter motor. The engine model includes forces due to inertia of reciprocating and rotating parts of engine, gas pressure, frictional loss while starter motor dynamics is determined by the motor's torque versus speed behavior. The numerical results of the analysis when compared with the experimental results showed close correlation.Engine starting by three models of starter motor is presented for a given battery. Effect of different parameters like engine inertia and reduction ratio between engine and starter motor is described. It is shown here how this analysis can be effectively used as a first step by an engine designer for determining a suitable starter motor characteristic and its related transmission parameters.