The lateral compositional distribution of indium content on InGaAs sample surface, which was grown on InP substrate by metal organic vapor phase epitaxy (MOVPE), was investigated by high resolution X-ray diffractometry (HRXRD). It was found that the In content increases exponentially in the gas flow direction. Based on the X-ray results, a general simplified model, which analyses the lateral composition distribution on the sample surface for a ternary alloy grown in a horizontal MOVPE reactor, is proposed according to a chemical chain reaction and mass transport theory. This model takes into account the thermal decomposition and vapor-solid interface reaction in the MOVPE process. Furthermore, according to this model, some proposals were suggested in order to improve the compositional uniformity in a ternary alloy.